Contribution margin

Contribution margin is what remains of an order once every variable cost of fulfilling it is gone: product cost, shipping and fulfillment, payment and platform fees, and an allowance for returns. It is the money available to pay for advertising and, after that, to be profit. Nothing else in ecommerce advertising is as load-bearing.

Almost every target traces back to it. Breakeven ROAS is one divided by contribution margin as a percentage. Breakeven CPA is contribution margin in currency per order. POAS bidding sends it, or a banded approximation of it, to the platforms as the conversion value. Get it wrong by five points and every downstream target inherits the error, silently, because none of the dashboards know the input was wrong.

Work an order to the bottom. A $150 order: $60 product cost, $11 pick, pack and shipping, $4.35 in payment fees at 2.9%, and a returns allowance of $9 at a 6% return rate on the gross margin at risk. Variable costs total $84.35, so contribution margin is $65.65, or 43.8%. From that single figure: breakeven CPA is $65.65 and breakeven ROAS is 2.28. The two lines most often skipped in a hurried version are fees and the returns allowance, and here they account for over $13 — enough to move breakeven ROAS from 2.05 to 2.28.

The common failure is computing it once, account-wide, from the flagship product, and then governing the whole catalog with the result. Margins vary across a catalog by twenty points or more, so one number is too strict for the profitable range and too generous for the thin one. The failure conceals itself in aggregate reporting: total performance can land exactly on target while a profitable half quietly subsidizes a half that loses money on every order.

Bands beat precision. Three or four margin bands across the catalog capture nearly all of the available signal, and they are achievable this quarter, whereas per-SKU landed cost including freight and duty is a project that competes with everything else finance is doing. The error you are removing is tens of points wide; the error banding leaves behind is a few points.

Rebase it quarterly with finance in the room, and treat it as a number the business owns rather than one marketing estimates. Supplier costs move, carrier rates move, return rates move with season and product mix, and a margin figure set in January misprices every decision made against it by autumn. The one habit that prevents most of this: whenever a target is written down, write the contribution margin it was derived from next to it.