Interchange is the wholesale rate the card issuer charges the merchant's acquirer on every transaction. It is set by the card networks (Visa, Mastercard, Amex, Discover) and published in detailed rate tables. Every merchant fee on every gateway is interchange plus something.
A card transaction has four parties: the cardholder, the issuing bank (issuer), the merchant, and the merchant's acquirer (which is sometimes also the gateway). The flow of fees is:
The cardholder's bank is paid the interchange portion to compensate it for funding the transaction, taking the credit risk, and handling fraud chargebacks. The network is paid the assessment for running the rails. The acquirer keeps the margin.
Visa and Mastercard publish their interchange tables openly. The Visa USA table runs to dozens of pages, covering hundreds of card-product-and-merchant-category combinations. The two main published tables are:
When Stripe charges 2.9% + $0.30, it is collecting interchange (1.65% typical blended), passing it through to the issuer, paying the network 0.13% assessment, and keeping ~1.10% + $0.30 as its margin. The merchant sees one number. The acquirer sees the stack.
“Interchange-plus pricing simply makes the stack visible. Flat-rate hides it; the merchant pays the same regardless of card mix.”
The implication is that a merchant whose card mix is heavy on regulated debit (low interchange) is subsidising one whose mix is heavy on premium rewards credit (high interchange) when they share a flat rate. Interchange-plus avoids that cross-subsidy.