> Credit card interchange runs up to about 2.9%, but that's only on the cards with the most benefits
Repeating what I asked in another comment, it's important to know what percentage of cards have those benefits. If most credit cards in the U.S. and Canada have cash back, miles, or rewards, then it follows that merchants pay 2-3% on most transactions. In my experience in the U.S. and Canada, almost everyone seems to use a cash-back or reward-type credit card.
Repeating what I asked in another comment, it's important to know what percentage of cards have those benefits. If most credit cards in the U.S. and Canada have cash back, miles, or rewards, then it follows that merchants pay 2-3% on most transactions. In my experience in the U.S. and Canada, almost everyone seems to use a cash-back or reward-type credit card.