The gap between a product’s gross margin and what actually lands in your bank account is where most Amazon businesses fail. Referral fee, FBA fulfilment, storage, returns, inbound shipping and advertising each take a slice, and several of them scale with price rather than with cost. This guide walks through a full per-unit calculation so you can see the number that matters.
Build the per-unit cost stack
Work from the selling price downward. For a product selling at $29.99:
| Line item | Amount | Notes |
|---|---|---|
| Selling price | $29.99 | Inc. any promotional discount |
| Landed unit cost (product + freight + duty) | −$7.50 | Freight and duty are part of COGS, not overhead |
| Referral fee (15%) | −$4.50 | Percentage of selling price |
| FBA fulfilment fee | −$5.50 | Size and weight tier dependent |
| Inbound shipping to FBA | −$0.60 | Often amortised and forgotten |
| Storage (monthly, amortised) | −$0.35 | Rises sharply in Q4 |
| Returns & refunds (~5%) | −$1.50 | Includes unrecoverable fees |
| Contribution before ads | $10.04 | 33.5% of selling price |
| Advertising (target 10% ACOS) | −$3.00 | Often 15–25% at launch |
| Net profit per unit | $7.04 | 23.5% of selling price |
The headline “we make 75% gross margin” ($29.99 − $7.50) collapses to 23.5% net once the stack is complete. Sellers who price from gross margin routinely end up with a business that grows revenue and loses money.
Run your own figures with the Amazon Profit Calculator, or use the Ecommerce Profit Calculator for other channels.
The fees that scale with price
Referral fees are a percentage of the selling price, so raising your price to protect margin also raises the fee in absolute dollars — but it improves the ratio, because the fixed-ish costs (fulfilment, storage, inbound) stay flat. This is why a $2 price increase on a $30 item can move net profit by more than $2: the fulfilment fee is unchanged while revenue rises. Model the change before assuming it will cost you the Buy Box.
The reverse is brutal. A $2 discount to win the Buy Box removes $2 of revenue, cuts the referral fee slightly, and leaves every fixed cost intact — so it can remove most of the $2 from profit, not just a fraction.
Advertising: the number that decides everything
ACOS (advertising cost of sale) is ad spend divided by ad-attributed revenue. The break-even ACOS equals your contribution margin before ads — in the example above, 33.5%. Spend above that on incremental sales and you are buying revenue at a loss.
That does not mean high ACOS is always wrong. At launch, paying above break-even buys rank, reviews and organic sales that later reduce the need for ads. The discipline is to know your break-even number and to treat launch spend as customer acquisition with a planned exit, rather than as a permanent cost structure. New listings commonly run 25–40% ACOS before settling toward 10–15%.
Storage and the Q4 trap
Storage is the fee most likely to break a plan that looked profitable, because it varies through the year and scales with how much stock sits unsold. Amazon’s monthly storage rate increases in the peak season, typically running for the fourth quarter, and inventory that does not sell through accumulates cubic-foot charges month after month.
Aged inventory attracts an additional surcharge once it has been in the warehouse beyond a set period, which means slow-moving stock gets progressively more expensive to hold. A product with a healthy 25% margin can turn negative if it sits long enough, and the loss is invisible in the per-unit calculation because it accrues over time rather than per sale.
The practical implication is that sell-through rate belongs in the profitability model, not just in the operations plan. If you are shipping a three-month supply and only selling two months of it, the third month’s storage plus the aged surcharge come straight off the margin of the units that did sell. Conservative planning means shipping smaller, more frequent replenishments even when the per-unit freight cost is higher — the freight premium is usually far smaller than a quarter of storage on dead stock.
Costs that sit below unit profit
Per-unit profit is not business profit. Below it sit fixed costs that must be covered by volume: subscription or professional selling plan, photography and listing creation, product liability insurance, software, samples and inspection, and the working capital tied up in inventory. Sellers often discover that a product with healthy unit economics still loses money at low volume, because two to three months of stock sitting in a warehouse absorbs more cash than the margin generates.
Inventory velocity matters as much as margin. A 20% margin turning six times a year beats a 40% margin turning once, both in cash terms and in exposure to a price war.
Key Takeaways
True FBA profit is selling price minus landed cost, referral fee, fulfilment, inbound, storage, returns and advertising — a 75% gross margin can become roughly 23% net. Know your break-even ACOS (your pre-ad contribution margin) and treat launch overspend as acquisition with an exit plan. Remember that fixed costs and inventory turns sit below unit profit and decide whether the business actually makes money.