Psychological Pricing on Amazon: What Works and What the Algorithm Ignores
Last updated: September 2026
Amazon's Buy Box algorithm compares prices as numbers. It evaluates $9.97, $9.99, and $10.00 as 9.97, 9.99, and 10.00 respectively. It does not know that human buyers perceive $9.99 as closer to $9 than to $10. The charm effect is invisible to the algorithm.
This does not mean psychological pricing is irrelevant on Amazon. It means the benefit occurs in a different place: after you win the Buy Box, not in winning it. The buyer who sees $9.99 in the Buy Box after your repricer has positioned you there converts at a higher rate than the buyer who sees $10.00. The research on this is consistent across decades and product categories.
The practical application for automated repricing: set your ceiling at a charm price point. Let the repricer compete between the cost-based floor and the charm ceiling. When competitive conditions allow the repricer to raise prices toward the ceiling, the buyer sees the psychologically effective price rather than a round number that captures slightly less of the available margin.
TL;DR: Charm pricing does not affect Buy Box allocation. Amazon's algorithm compares landed prices as numbers with no psychological weighting. Charm pricing affects conversion rate after winning the Buy Box, because the human buyer sees the price. The repricing application: set your maximum price (ceiling) at the nearest 9-ending price above the Keepa 90-day historical high. The repricer finds the competitive floor within that ceiling. When it reaches the ceiling, the buyer sees $9.99 rather than $10.00, and converts at a measurably higher rate.
What psychological pricing is and where it originated
Psychological pricing is the practice of setting prices at specific points that influence how buyers perceive value. The most studied form is charm pricing: prices ending in 9 or 99, where $9.99 is perceived as meaningfully cheaper than $10.00 despite a difference of one cent.
The research basis:
The left-digit anchoring effect was formalised in Thomas and Morwitz's 2005 study in the Journal of Consumer Research, "Penny Wise and Pound Foolish." The study demonstrated that buyers process prices by anchoring on the left-most digit: $9.99 anchors on 9 (not 10), $19.99 anchors on 19 (not 20). The perception of the $9.99-to-$10.00 gap is cognitively larger than the $9.00-to-$9.99 gap, despite both being $0.01 differences. This asymmetry is consistent across cultures and price ranges.
Anderson and Simester (2003) tested charm pricing in a controlled field experiment with a clothing mail-order catalogue. Identical items were listed at $34, $39, and $44 across separate catalogue versions. The $39 version (the charm price) outsold the $34 version by approximately 34%. The higher-priced charm point sold more units than the lower round price.
The three common psychological pricing forms:
Charm pricing (9-endings): $9.99, $19.99, $49.99. The most researched and the most directly applicable to Amazon ceiling-setting.
Bundle pricing: Presenting combined prices in ways that make the marginal unit feel free or near-free. ("Buy 2 for $18" versus two individual listings at $9.99.) Primarily relevant for Amazon multi-pack listings rather than repricing configuration.
Prestige pricing: Round numbers ($100, $250) that signal quality. Research by Stiving and Winer (1997) showed round numbers perform better for premium products where buyers use price as a quality signal. This applies to private label at the high end of a category.
For Amazon FBA sellers using automated repricing, charm pricing and its ceiling application are the most directly actionable form.
How Amazon's Buy Box algorithm evaluates price and what it ignores
Amazon's Buy Box algorithm evaluates the landed price as a number in a comparison. $9.97 < $9.99 < $10.00. No human perception layer exists in this evaluation. A seller at $9.97 has a pricing advantage over one at $9.99, and the algorithm knows this at the numerical level only.
What the algorithm evaluates:
The algorithm takes the landed price (item price plus shipping cost for the offer) and compares it against eligible competing offers alongside fulfillment method, seller metrics, and inventory level. Price is factored as a number in this comparison, not as a perceptual category.
For two FBA sellers with equivalent metrics competing on the same listing: the lower landed price wins more Buy Box rotation time. $9.97 wins more time than $9.99. $9.99 wins more time than $10.00. The algorithm knows only the numerical order.
What the algorithm does not evaluate:
The algorithm has no model of human price perception. It does not know:
That $9.99 anchors on 9 rather than 10 for most buyers
That buyers resist crossing certain psychological thresholds
That a price ending in .99 feels like a significantly different value category from the same price rounded up
The implication for repricing rules:
An undercut rule set to "price $0.01 below the lowest competitor" does not produce charm prices automatically. If the lowest FBA offer is $10.00, the rule sets your price at $9.99, which happens to be a charm price, but by arithmetic coincidence rather than strategic intent. If the lowest offer is $10.02, the rule sets your price at $10.01, not a charm price.
The Buy Box algorithm treats all these prices identically except as numbers in a comparison. The human buyer does not.
Where psychological pricing does work on Amazon: after you win the Buy Box
Psychological pricing affects human buying behaviour, and the human buyer appears after the algorithm has already decided who holds the Buy Box. The price displayed in the "Add to Cart" button is seen by a human. That human is subject to the same left-digit anchoring effects documented in the research.
The conversion rate effect:
When a buyer arrives at a product listing where you hold the Buy Box, they see the displayed price. The charm price effect on conversion rate is consistent across the research literature: prices ending in 9 outperform prices $0.01 higher (round numbers) in conversion rate for most product categories.
The magnitude varies by category, price range, and product type. The effect is strongest:
At price points where buyers use price as a primary decision signal (commodity products, value-positioning)
When the product crosses a whole-number threshold ($19.99 vs $20.00 more than $19.99 vs $19.98)
When buyers have moderate category familiarity. High-familiarity buyers anchor on reference prices. Low-familiarity buyers are more susceptible to charm effects.
The threshold-crossing effect:
The charm price effect is strongest at round-number thresholds. $19.99 versus $20.00 is a larger perceptual gap than $9.49 versus $9.50, because $20 represents a stronger psychological barrier than $9.50. Setting your ceiling at $19.99 rather than $20.00 captures this effect at the most commercially significant moment: when your repricer reaches the ceiling.
Where this matters in repricing:
A seller whose repricer uses ceiling-hunt rules (raises prices toward the ceiling when the competitive set thins) reaches their ceiling price periodically, when a competing seller stocks out, exits the listing, or reprices upward. At these moments, the displayed price in the Buy Box is the ceiling price. Setting the ceiling at $19.99 rather than $20.00 means the buyer who arrives during these peak-price windows sees the charm price rather than the round number.
Book a Demo, configure charm-price ceilings and cost-based floors in Repricer.com before the next competitive event on your listings.
How to use charm price points as your repricing ceiling
The ceiling in a repricer is the maximum price the rule sets, reached when competitive conditions allow higher pricing. Setting the ceiling at a charm price point means that the human buyer who arrives when your repricer is at its maximum sees a psychologically effective price rather than a round number.
The ceiling-setting process:
Pull the 90-day Buy Box price historical high for the ASIN from Keepa
Round up to the nearest 9-ending price above that high
Set this as the ceiling (maximum price) in Repricer.com
The rounding logic:
Keepa 90-day high: $14.70 → nearest 9-ending above: $14.99
Keepa 90-day high: $21.50 → nearest 9-ending above: $21.99
Keepa 90-day high: $29.10 → nearest 9-ending above: $29.99
In each case, the ceiling is slightly above the historical maximum. This is correct: the historical high shows the price at which the listing has sustained Buy Box activity previously. Setting the ceiling above that level allows ceiling-hunt rules to probe whether the current competitive environment supports a marginally higher price.
Why .99 specifically over .95 or .97:
The research is strongest for 9-endings (not 5 or 7 endings) specifically because the left-digit anchoring effect is maximised when the price is as close to the threshold as possible without crossing it. $9.99 is as close to $10 as a 9-ending price gets, maximising the anchoring effect on the 9 rather than 10. $9.95 has a slightly weaker effect because some portion of buyers will round $9.95 to $10 in their mental arithmetic.
This matters most at whole-number thresholds: $9.99, $19.99, $29.99, $49.99. It matters less between thresholds: the difference between $14.99 and $14.95 is smaller than the difference between $19.99 and $20.00.
Setting minimum and maximum prices around psychological thresholds
The floor (minimum price) is calculated from costs, not from psychological price points. The ceiling (maximum price) is set at a charm price point. This separation is important: mixing psychological intuition into the floor calculation produces margin errors.
The floor: cost-based, not psychology-based
The minimum price in a repricer must reflect the cost structure of the product, not what "feels right" or what ends in 9. A floor set at $8.99 because it sounds better than $9.11 is wrong if the cost-calculated minimum is $9.11. The repricer will sell at $8.99 and erode margin on every sale.
The correct floor formula: (sourcing cost + prep cost + FBA fee) divided by (1 minus referral fee rate minus target margin rate). The output is a calculated number. It does not end in a psychologically tidy digit by design.
For the FBA fee inputs, use the current fee schedule from the Amazon seller fees guide. For automatic floor recalculation when costs change, Net Margin Repricing keeps the floor current without manual updates.
The ceiling: charm-price setting
Set the ceiling at the charm price calculation described in Section 4. The ceiling is where psychological pricing applies, because the ceiling represents the maximum a human buyer sees when your repricer is in its most favourable competitive position.
The repricing range:
Repricer operates between floor and ceiling. Within this range, the rule type determines where prices settle:
Match rules: hold the competitive Buy Box price (wherever competitors price between floor and ceiling)
Ceiling-hunt rules: raise toward the ceiling when competitors thin out
Position-targeting: target a specific Buy Box share within the range
The buyer sees whatever price the repricer has settled at within this range. The charm effect at the ceiling is the strongest, because that is where the price is highest and the conversion rate impact is most commercially significant.
Examples: repricing between $9.49 and $9.99 on a shared ASIN
These examples use realistic price ranges to illustrate how the floor-to-ceiling repricing band interacts with psychological price points.
Example 1: Low-price commodity product
Product: household accessory, small standard FBA, sourced at $3.20.
Floor calculation:
Sourcing cost: $3.20
Prep cost: $0.35
FBA fee: $3.18
Referral fee: 8%
Target margin: 20%
Floor = ($3.20 + $0.35 + $3.18) ÷ (1 minus 0.08 minus 0.20) = $6.73 ÷ 0.72 = $9.35
Keepa 90-day high for the listing: $9.70. Ceiling: nearest 9-ending above $9.70 = $9.99
Repricing range: $9.35 to $9.99.
During normal competition (3 FBA sellers on the listing), the match rule holds near $9.49 (the competitive Buy Box price). A buyer who arrives sees $9.49, a charm price arrived at by competitive dynamics rather than deliberate setting, but still in the charm-price zone.
When one seller stocks out, the ceiling-hunt rule raises toward $9.99. A buyer who arrives during this window sees $9.99, the charm ceiling, rather than $10.00. Conversion rate at $9.99 is measurably higher than at $10.00 for this price range.
Example 2: Mid-price product with psychological threshold
Product: home goods accessory, medium standard FBA, sourced at $12.50.
Floor calculation:
Sourcing cost: $12.50
Prep cost: $0.50
FBA fee: $5.06
Referral fee: 8%
Target margin: 20%
Floor = ($12.50 + $0.50 + $5.06) ÷ 0.72 = $18.06 ÷ 0.72 = $25.08
Keepa 90-day high: $29.20. Ceiling: $29.99 (the threshold immediately below $30, the strongest charm threshold in this price range)
The $29.99 ceiling is 2.7% above the Keepa historical high. A thin-competition window that allows the repricer to reach $29.99 rather than a round $30.00 produces better conversion from the human buyers who arrive during those windows, at equivalent cost.
Key Takeaways
Amazon's Buy Box algorithm compares prices as numbers. Charm endings have no effect on Buy Box allocation. $9.97 beats $9.99 in an identical algorithm comparison.
Charm pricing affects human buyers, and human buyers arrive after the algorithm has already awarded the Buy Box. The conversion rate effect is real, consistent, and documented in peer-reviewed research.
The practical application is the ceiling. Set your maximum price at the nearest 9-ending above the Keepa 90-day historical high. When the repricer reaches the ceiling, the buyer sees a charm price.
The floor must be cost-based, not psychology-based. A floor set to a round number that feels tidy is wrong if the cost calculation produces a different number. Margin erosion from a wrong floor costs more than any charm-price benefit at the ceiling.
The threshold effect is strongest at round-number boundaries. $19.99 versus $20.00 captures more of the charm effect than $14.99 versus $15.00. Prioritise ceiling-setting at .99 when the Keepa data puts the ceiling near a round-number threshold.
Action Plan
For each active repricing ASIN, pull the Keepa 90-day Buy Box price historical high.
Identify the nearest 9-ending above the Keepa high for each ASIN. If the Keepa high is $14.70, the charm ceiling is $14.99. If it is $21.50, the charm ceiling is $21.99.
Update your ceiling (maximum price) in Repricer.com to the charm ceiling for each ASIN. This is a one-time update per ASIN, repeated quarterly when the Keepa 90-day window rolls forward.
Confirm your floors are correctly calculated from cost inputs, not from round numbers or psychological estimates. Use the formula: (sourcing cost + prep cost + FBA fee) ÷ (1 minus referral fee rate minus target margin rate). The 10-point repricing configuration audit verifies floor accuracy across the catalogue.
Enable ceiling-hunt rules on ASINs with 1 to 3 active FBA sellers. Ceiling-hunt rules raise toward the charm ceiling when the competitive set thins, producing the charm effect for buyers who arrive at peak-price moments.
Prioritise ceiling-setting at round-number thresholds. ASINs where the Keepa high is close to $10, $20, $30, $50, or $100 benefit most from charm ceiling placement at $9.99, $19.99, $29.99, $49.99, or $99.99 respectively.
Frequently Asked Questions
1. Does psychological pricing work on Amazon?
The answer depends on where you apply it. Psychological pricing does not affect Amazon's Buy Box algorithm, which compares landed prices as numbers with no perception layer. $9.97 wins more Buy Box time than $9.99 in a direct comparison. Psychological pricing does affect human buyers who arrive after the algorithm has awarded the Buy Box. A buyer who sees $9.99 in the "Add to Cart" button converts at a higher rate than one who sees $10.00, because the left-digit anchoring effect documented by Thomas and Morwitz (2005) applies to the human perception of the displayed price.
2. Will charm pricing affect my Buy Box win rate?
No. The Buy Box win rate is determined by the algorithm's comparison of landed prices and other eligibility factors. The algorithm treats $9.99 and $10.00 as numbers 9.99 and 10.00, with 9.99 winning more Buy Box time than 10.00 in an otherwise equal comparison. Setting prices at charm endings does not give a Buy Box advantage over non-charm endings at the same price level. The advantage from charm pricing is in conversion rate (human buyers) rather than Buy Box allocation (algorithm).
3. How do I combine psychological pricing with automated repricing?
Set your ceiling at a charm price point and let the repricer handle everything between the cost-based floor and the charm ceiling. The ceiling is where psychological pricing matters, because it is the maximum price the repricer sets when competitive conditions allow the highest price. When a ceiling-hunt rule reaches $9.99 rather than $10.00, the buyer who arrives at that moment sees the charm price. The floor remains a cost-calculated number, do not adjust the floor for psychological reasons, as this produces margin errors.
4. Should my repricing maximum price end in .99?
Yes, if your Keepa 90-day historical high supports a ceiling at or above the nearest 9-ending. The formula: take the Keepa 90-day Buy Box price high for the ASIN, identify the nearest 9-ending above it (not below it), and set that as the ceiling. If the Keepa high is $14.70, set the ceiling at $14.99 (not $14.70 exactly and not $15.00). The .99 ceiling captures the charm effect at the repricer's highest price moments without setting the ceiling above what the listing has historically supported.
Book a Demo, configure charm-price ceilings from Keepa historical data and cost-based floors from your actual unit economics in Repricer.com's Amazon Repricer.
