To get the fully nuanced picture:
-If you wander around to a grocery store bakery, and eat one of the donuts before getting to checkout, you have the right to settle your debt with cash.
-Otherwise, the store can force you to pay with their preferred method, or else will return the merchandise to the shelves.
Yeah, that's a good point. I mostly wanted to convey a payment after use of the product or service. A better example might be paying your mechanic after he finishes fixing your car.