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What comes off a sale

• The marketplace's commission. A percentage of the sale, and on some platforms a percentage of the postage too — which catches people out, because the postage was never yours to keep.
• Payment processing. Usually a percentage plus a fixed amount per transaction. The fixed part is what makes cheap items disproportionately expensive to sell.
• Listing fees, where they exist. Small, but charged per listing and often per renewal — so they are paid on items that never sell as well as ones that do.
• Optional promotion. Offsite ads and promoted listings take a further cut, and on some platforms you can't opt out once your sales go above a certain level.
• Postage you absorbed. If you offer free delivery, you are paying the postage yourself, so count it here with everything else.
• Packaging. Box, filler, tape, label. Pennies each, and the only line here that scales exactly with units sold.

Why the fixed fee changes what is worth selling

Percentage fees are neutral about price — they take the same share of a £5 sale as a £50 one. A fixed per-transaction fee is not. It is a much larger share of a small sale, which is why low-value items can look profitable on cost and still barely wash their face once selling costs are applied.

That fixed fee is the reason behind advice you have probably heard before: bundle small items, set a minimum order, and don't list a £3 keyring on its own.

Work it out on the price, not in the cost

It is tempting to add an estimated fee percentage to your cost per unit and forget about it. Don't. Cost per unit is what it costs to make the item, and that shouldn't change just because you list it on a second platform with a different commission.

Keep the two separate: cost tells you the floor, selling costs tell you what a given price actually returns on a given channel. The same item can then be priced differently in different places without your cost figures drifting to match.

The number worth knowing

For each channel you sell through, work out what proportion of a listed price you keep. Once you have it, pricing becomes a division rather than a guess: if you need £12 to clear your floor plus margin and the channel keeps 15%, you need to list at about £14.10, not £12.

Most sellers find they keep less than they thought.

A note on tax and thresholds

Selling costs are business expenses, so they reduce what you are taxed on — but only if they are recorded. Marketplace statements are easy to forget to download, and piecing a year of them together later is miserable. Save them every month.

This is general information about running the numbers, not tax advice; thresholds and rules differ by country and change.

Where MakerProof fits

Cost per unit — filament, waste, machine time and the failure-rate division — comes from prints you have logged, and marketplace fee presets come with your country. So the floor is derived rather than estimated, and the fee deduction is applied to the price where it belongs.

How to price a 3D print puts the whole sequence together, or start free.