Contribution margin per unit — after commission, fulfilment, returns, ads and GST — is the number that decides whether a SKU should exist online at all.

Take your selling price. Subtract referral commission, closing fee, fulfilment or shipping, expected returns cost, allocated ad spend and GST. What’s left is contribution margin per unit — and if it’s negative, the platform is wrong for that SKU regardless of its traffic.

Returns are the cost sellers underestimate most. A high return rate in apparel changes the entire economic model, and RTO costs on prepaid-light categories can quietly erase a quarter’s profit. Track returns per SKU: they usually concentrate in a small number of listings.

Build the model per SKU, per platform, before you list. Some products simply cannot be profitable on a given marketplace once every cost is accounted for — and it is far cheaper to learn that in a spreadsheet than in your settlement report.

Once the model exists, every later decision gets easier: which SKUs to push with ads, which to hold, where your break-even ACoS sits, and when a discount ladder stops being marketing and starts being margin leakage.