Interchange
The fee set by the card networks and paid to the issuing bank on every transaction.
Interchange is the portion of a card transaction fee that goes to the bank that issued the customer's card. It is set by the card networks, not by your processor, and no provider can discount it, which makes it the true floor under any rate you are quoted.
The rate varies with how the payment was made and what card was used. Card-not-present transactions cost more than in-person ones because fraud risk is higher. Commercial, corporate and rewards cards carry higher interchange than standard consumer debit, so a business selling to other businesses will see a higher blended cost than a consumer retailer at identical volume.
This is why interchange-plus pricing is more transparent than a single blended rate. Under interchange-plus you see the network cost and your provider's margin separately. Under a blended rate both are combined into one number, so a provider can absorb an interchange decrease without passing it on and you cannot tell.
When comparing quotes, compare the margin over interchange rather than the headline rate. Two providers quoting different blended rates may have identical margins and simply different assumptions about your card mix, and the cheaper-looking quote can end up costing more once real transactions arrive.