How to Use This Tool
The number that kills Amazon businesses is not the referral fee, which everybody knows about. It is the accumulation: referral, then fulfilment, then storage, then the units that come back, then the advertising that was needed to get the sale in the first place. Each is individually survivable. Together they routinely turn a product that looked like a 40% margin into one that loses money on every unit at scale.
The four inputs that decide everything
Start with selling price and landed unit cost. Landed means the price you paid the factory plus freight, duty and any inspection — not the invoice figure. Sellers who use the invoice figure here are the ones who discover their real margin at the end of the quarter.
Category sets the referral fee, which is a percentage of the selling price and typically runs 8% to 17%. Note the consequence: raising your price does not raise your profit by the full amount, because the referral fee scales with it.
Size tier sets the fulfilment fee, and this is where most product research goes wrong. Fulfilment is a flat charge per unit based on dimensions and weight, so it is brutal on cheap, bulky or heavy goods. A product crossing from Large Standard into Small Oversize can lose several dollars per unit without changing a thing about the customer's experience.
The costs hidden behind the expander
Storage is charged per cubic foot per month and jumps by roughly three times in the October to December peak. Enter how long stock genuinely sits, not how long you hope it will. Returns cost more than the refund: you lose the fulfilment fee, frequently the unit itself, and the return processing. Advertising is entered per unit sold rather than as a budget, because that is the figure that determines whether a sale was worth making.
Reading the four metrics
Net margin is profit over selling price. Below 15% there is no room for a fee change or a price war. ROI on cost is profit over the cash you tied up, which is the number that matters when your constraint is working capital rather than shelf space; experienced sellers target 100% or better. Break-even ACoS is the advertising cost of sale at which profit reaches exactly zero — run campaigns above it and you are buying rank, not income, which is a legitimate strategy but should be a decision rather than an accident. Break-even price is the lowest price you can match in a competitor's race to the bottom before you are subsidising them.
Assumptions worth stating
Fee schedules are entered as current published rates and Amazon revises them regularly, usually upward and usually in January. Treat the output as a decision tool for comparing products, and verify the exact fees for a specific ASIN in Seller Central before committing to a purchase order.