The denominator makes the difference
Markup compares the amount above a chosen cost base with that cost base. Margin compares profit with revenue. If an item costs $40 and sells for $60, the $20 difference is a 50% markup on cost but only a 33.3% margin on revenue. Both statements can be correct, provided the same costs are included and the denominator is named.
In an online shop, the difference between selling price and product cost is rarely the final profit. Payment fees, marketplace charges, packaging, fulfillment and advertising can consume part of it. Call a number a product-cost margin if it excludes those costs; do not silently present it as take-home profit.
Build a complete example
Assume a $50 sale with no separately collected shipping. Product cost is $20, fulfillment is $5, advertising is $4, and the illustrative fee schedule is 10% of revenue plus $0.50. Fees are $5.50, leaving $15.50 after these modeled costs. The modeled margin is $15.50 ÷ $50 = 31%.
Comparing the same $50 selling price only with the $20 product cost gives a 150% markup and a 60% product-cost margin. Neither reflects the additional $14.50 in fees, fulfillment and advertising. This is why a large markup can coexist with a much smaller final contribution.
Solve for a target margin
With total non-percentage costs C, fixed transaction charge f, percentage fee r and desired margin m, the simplified target revenue is (C + f) ÷ (1 − r − m). All percentages must be decimals, and the denominator must be positive. This assumes the percentage fee and target margin use the same revenue base.
In the example, C is $29, f is $0.50, r is 0.10 and a 30% margin target is 0.30. Revenue is $29.50 ÷ 0.60, or approximately $49.17. Rounding and fee minimums can change the exact cent required. If advertising is a percentage rather than a fixed dollar amount, include it in the percentage side of the equation instead.
Read the result honestly
Use the e-commerce calculator to enter product cost, fulfillment and advertising separately. Then test a sale price slightly above and below your target. The calculator’s net profit is a modeled transaction result, not an audited business profit. Monthly subscriptions, returns, owner labor and income taxes need separate treatment unless you have deliberately allocated them.
Keep your cost definition beside every margin target. Comparing a margin that includes fulfillment with one that excludes it is likely to mislead you even when both calculations are arithmetically correct.