What is Contribution Margin?

Contribution margin is what a unit contributes towards advertising, overheads and profit once Amazon has taken its fees and the goods have been paid for. It is the number every advertising decision is priced against, because advertising is the variable you are steering and the other costs are already committed.

The ladder

Profit is best read as rungs rather than one netted figure, so that a mistake on any single line stays visible:

CM1 = Net revenue − Amazon fees      (what Amazon leaves you)
CM2 = CM1 − COGS                     (contribution before ads)
CM3 = CM2 − Advertising              (contribution after ads)

Worked example

Suppose a unit sells for £30 with £8 of Amazon fees and a £7 landed cost, and the order took £6 of advertising.

CM2 is the bid ceiling. It is the most an incremental sale could ever be worth paying for, and dividing it by revenue gives break-even ACoS.

The fee double-count

The most expensive arithmetic mistake in this area is subtracting fulfilment fees twice. In Amazon's reporting an "Amazon fees" total normally already contains the fulfilment fee, so taking a separate fulfilment line off as well understates margin by the fulfilment share of revenue. That biases break-even downwards, which under-bids every keyword measured against it. Reading a rung of the ladder, rather than rebuilding it from components, makes the error impossible to express: at CM2 there is no fee term left to subtract. A quick check is to remove the fulfilment fee from the fee total and see whether the residual lands in the referral fee band for that category. If it does not, something is being counted twice.

Decide once where refunds and promotions sit

Refunds and seller promotions can be netted out of revenue or listed as costs. Either convention works. Doing both, in different reports, is how two teams end up with two profit figures for the same week. The third failure is quieter: a cost that enters no rung at all, while the wording beside the number says it is included. A line belongs in the ladder or it belongs nowhere, and the copy has to match the arithmetic. Fix the convention, write it down, and check the ladder still reconciles to a settlement.

Contribution is not net profit

Contribution margin deliberately excludes fixed overheads: software, staff, storage that would be paid whether or not this unit sold. That is what makes it the right basis for a bid and the wrong basis for a valuation. For the whole-business figure, see net margin.