Start with the invoice
Payment acceptance can involve several separately described charges. Interchange is not synonymous with the merchant’s full acceptance price. Mastercard explains that it is one component of the merchant discount rate; the merchant’s arrangement with its acquirer determines the commercial bill it actually receives.
Choose a denominator
A fee percentage needs a defined volume base. Gross processed value, settled value and successful purchase value can differ. When comparing plans, also inspect fixed charges and which events attract them. A low headline percentage can tell very little about the cost of small transactions.
A basket example
Under a fictional price of 1% plus €0.10 per successful payment, a €5 basket costs €0.15, or 3%. A €100 basket costs €1.10, or 1.1%. These are arithmetic examples, not market quotations; basket size changes the effective rate under exactly the same tariff.
Keep commercial detail visible
Model your actual mix of basket sizes, currencies and supported payment types. Identify the treatment of refunds, disputes and unsuccessful attempts in the relevant contract. Reconcile invoice lines to events rather than attributing every unexplained deduction to interchange, and avoid assuming one published table describes every merchant arrangement.