Contribution margin is the per-order profit signal — the dollars left after every variable cost of producing, processing, and delivering the order is subtracted from revenue. It diverges from gross margin in what it subtracts: gross margin stops at COGS; contribution margin keeps going through every cost that scales with shipping an additional order.
The DTC variable-cost stack above the contribution line is COGS, payment-processor fees (a per-transaction percentage plus a fixed cent amount), pick-pack-ship, inbound freight allocated per unit, a returns and refund reserve sized to the brand’s own return rate, and merchant discounts. Three line items are commonly missed: the returns reserve (real cash going back out, not a COGS write-off), processing fees (treated as overhead when they are strictly variable), and the marginal cost of “free shipping” promos, where the brand absorbs the carrier rate on every order.
The link to ROAS is algebraic: breakeven ROAS = 1 / contribution margin %. A 30%-margin SKU breaks even at about 3.33, so a 3.0 ROAS on that SKU loses money after fulfillment. CAC is denominated in the same unit — LTV:CAC and payback period reduce to how many orders of contribution margin are needed to recover acquisition spend.
Contribution margin is the primitive that turns revenue ratios into profit signals. ROAS and AOV read differently against it, which is why it belongs alongside them on the dashboard rather than below them.