Are "prompt payment" and "payment on account" antonyms?
I know that there is a type of payment called "deferred payment". I think that it means "payment on account" but I'm not sure.

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Answer

We need context here to better understand.

Prompt payment usually means payment is due when you receive the bill.
Payment on account, I can't tell without context. But I think of a statement where they are itemizing previous payments.