Every paid click is a bet: the click costs a fixed amount, and pays out only when a visitor converts. Which means there's a precise ceiling on what a click is worth to you — and it's probably lower than your current bid.
Max CPC = profit per sale × conversion rate
If you clear £8.50 per sale after all costs and fees, and 2% of visitors buy, each visitor is worth £8.50 × 0.02 = 17p. Bid above 17p and you lose money on average with every click, however good the traffic feels. Bid at 17p and you break even — which, as with break-even ROAS, means working for free.
Profit per sale must be the post-everything number: base cost, shipping you absorb, listing fees, transaction percentage, payment processing — the platform charges those on ad-driven sales too. Conversion rate should come from your actual shop stats, not a benchmark; new shops without data can start with 1–2% as a conservative assumption and update monthly. If your true conversion rate is 1% rather than 2%, your max CPC just halved — this number dominates the answer, so measure it.
Set campaign bids at 50–70% of max CPC, not at it — the margin between bid and ceiling is your actual profit. Kill keywords whose real CPC drifts above the ceiling; no amount of "brand building" makes a structurally unprofitable keyword profitable. And when a platform's auto-bidding spends above your ceiling, that's not the algorithm knowing better — that's the ceiling doing its job by ringing an alarm.
Two levers raise max CPC: more profit per sale (pricing, cheaper base product) or better conversion (photos, reviews, listing copy). A shop that lifts conversion from 1% to 2% doubles what it can afford per click — which in auction terms means outbidding every competitor who didn't do the work. Margin quality is a marketing weapon.