Transaction fees are what payment providers keep from every charge: a percentage of the amount plus a fixed fee per transaction, at rates that differ by provider, card type and country. They are a per-order cost, which is why smaller baskets quietly pay a higher effective rate.
Two details decide whether the number is right: an order split across two payment methods is charged by both providers, fixed fee included, and providers do not hand their fee back when you refund.
The fixed part is why falling average order value quietly raises your effective fee rate: the same cents take a larger bite of a smaller basket, with no rate change anywhere to point at.
On Shopify Payments there is nothing to estimate: Shopify reports the fee it actually charged on each payment, and nouz books that figure. A fee rule, a percentage plus a fixed fee from a start date, prices only the payments that carry no reported fee, which in practice means PayPal, Klarna, manual methods and the rest. Where each charge shows up in Shopify's own reports is mapped in where the fees hide.
Because every store takes money through several gateways at different prices, the number that reaches a P&L is a blend; the Shopify payment fees calculator works that blend out from your own mix and shows which leg of the fee, the percentage or the fixed part, your basket size is exposed to.
