Most FBA sellers price by feel and get blindsided by the fee stack later. Take a $49.99 item costing $15 to make, with $2 in prep and packaging per unit: at a 15 percent referral rate, Amazon takes about $7.50 off the top. A two-pound standard-size item then costs roughly $4.00 to fulfill, plus somewhere around $0.45 a month in storage for a standard-size unit. Add it up and fees alone run close to $12 a unit, on top of the $17 you already spent on product and prep. That still leaves roughly $21 of profit per unit, a margin around 42 percent, but only because the numbers happened to work out. Change the weight, the category, or the price and that margin moves fast.
The fulfillment fee is the piece sellers most often get wrong, because it is not a flat rate, it scales with weight and whether the item is standard size or oversize. A one-pound standard item is cheaper to fulfill than a five-pound one; cross into oversize and the per-unit fulfillment cost jumps again. Referral fees add another layer of variation, running anywhere from about 6 percent in some categories up toward 45 percent in a few restricted ones, with 15 percent as a common middle ground for general merchandise. None of this shows up on the product listing page, which is exactly why so many sellers discover their real margin only after the first month of sales.
Selling 100 units a month at that same $21-per-unit profit works out to roughly $2,100 in monthly profit, or about $25,250 a year, before accounting for advertising, returns, or slow months. That is the number worth stress-testing before you commit to a product: raise the price two dollars, switch from oversize back to standard packaging, or negotiate the product cost down, and watch how much each lever actually moves your annual total instead of guessing.
The three fees stacked on every sale
Every unit you sell carries a referral fee (Amazon’s cut of the sale price, set by category), a fulfillment fee (what Amazon charges to pick, pack, and ship the order, driven mainly by weight and whether the item counts as standard or oversize), and a storage fee (a small monthly charge just for the inventory sitting in the warehouse). None of these are optional or negotiable per order; they are structural costs baked into the FBA model, and any profit calculation that ignores one of them will overstate your margin.
Why weight matters more than most sellers expect
Fulfillment cost is not proportional to price, it is tied to weight and size tier. A light standard-size item is inexpensive to ship; add a pound or two and the fee steps up noticeably, and crossing into the oversize tier changes the fee formula entirely rather than just scaling it. Two products priced the same but with different weights can end up with meaningfully different profit per unit purely because of what it costs Amazon to move them.
What counts as your true product cost
Product cost per unit should include what you paid your manufacturer, plus prep and packaging, plus anything you spend getting inventory into an Amazon warehouse. Sellers who only track the wholesale unit cost and forget prep, packaging, and inbound shipping consistently overestimate their margin, sometimes by several dollars a unit, which is enough to turn a real profit into a break-even product once the fee stack is added on top.
Finding your break-even price
Add your product cost, additional per-unit costs, referral fee, fulfillment fee, and storage estimate together and that total is your break-even price, the minimum you must sell at just to avoid losing money. Any price above that is where profit actually starts, so before committing to a supplier order, check that your planned selling price sits comfortably above this number rather than just a dollar or two over it.
How storage costs creep up over the year
The storage estimate here reflects routine monthly warehousing, but Amazon charges more during the fourth quarter when warehouse space is tightest, and inventory that sits unsold for many months can trigger additional long-term storage charges. Keeping inventory lean and turning over stock rather than over-ordering protects the margin you calculated at the start.
Deciding if a product is worth listing
Once you know your profit per unit and monthly volume, multiply them out to an annual figure and compare it against the time and cash tied up in that SKU. A product with a thin per-unit margin can still be worthwhile at high volume, while a product with a healthy-looking margin but very low sales velocity may not be worth the shelf space in your sourcing budget.