The SKU That's Costing You Money Every Time It Sells

Your best-selling SKU might be your most expensive mistake.

Shopify shows gross margin. It doesn't show what happens after ad spend, returns, and fulfilment eat through it.

Why Shopify gross margin hides unprofitable SKUs

Here's a scenario every DTC founder will recognise. You've got a product doing 68% gross margin. Looks healthy. Your team pushes ad spend behind it because it "converts well." It becomes your hero SKU — the one that gets the homepage banner, the retargeting budget, the email feature.

But when you factor in what it actually costs to sell that product — the ad spend per unit, the fulfilment cost, the return rate on that specific SKU, the payment processing fees — the contribution margin drops to -4%.

Negative four percent. Every unit you sell costs you money. And the more you promote it, the more you lose.

How to calculate true net margin per SKU

Meanwhile, sitting quietly in your catalogue is a product with 45% gross margin that nobody's paying attention to. It sells mostly through organic and email. Low return rate. The contribution margin is 31%. Every unit sold drops real cash into the business.

Shopify will never show you this. It can't. It doesn't know your COGS per SKU. It doesn't map ad spend to individual products. It has no concept of contribution margin. So its "Top Products" report ranks by revenue — which, in this scenario, puts the loss-making product at the top and the profitable one at the bottom.

Every founder looking at that report is getting a ranking that's potentially the inverse of what actually matters.

When to cut a top-selling product from your catalogue

The fix isn't complicated. You need three things Shopify can't give you: contribution margin per SKU (not just gross margin), profit ranking alongside revenue ranking, and ad spend per SKU mapped to profit per SKU. When you can see that you're spending £11 in ads to generate -£2 in profit on one product, and £4 to generate £18 on another, the reallocation decision makes itself.