The formula
fee = amount × % + fixed · to receive N net: charge (N + fixed) / (1 − %)
Where it comes from
Every processor charges the same way: a percentage of the amount plus a fixed sum per transaction, and sometimes a surcharge when the card is foreign or there is a currency conversion. What changes is the figure, and it changes by country, card type and volume: Stripe publishes a standard rate per region, Mercado Pago charges by how soon you want the money available, and both revise their terms without notice. That is why the percentages here are fields, not constants. The reverse calculation is the one most often forgotten: to receive a hundred net it is not enough to charge a hundred plus the fee, because the fee also applies to what you add. Solving the equation gives a little more.
How to work it out by hand
- Add the base percentage and any surcharges that apply
- Multiply the amount by that percentage and add the fixed fee
- Subtract the fee from the amount: that is what you keep
- To receive a net amount, add the fixed fee and divide by one minus the percentage
What is worth knowing
The fixed fee is what makes small sales expensive. At 2.9 % plus $0.30, a $100 payment loses 3.2 %, but a $5 payment loses 8.9 %: the fixed fee weighs as much as the percentage when the amount equals fixed over percentage, about $10 in that example, and below that the fixed fee dominates. It is why processors do not work for micropayments and why many shops set a minimum. Mercado Pago prices by when you want the money: releasing it immediately costs several points more than waiting thirty days, and each country has its own table. And a fee is not a tax-free cost: in several countries VAT is charged on top of it, added to the fee in Mexico and withheld separately in Argentina.