IPA: /ˈoʊvərˌpeɪmənt/
KK: /ˈoʊvərˌpeɪmənt/
A payment that is more than what is required or owed.
The company issued a refund for the overpayment made by the customer.
Overpayment → It is formed from "over-" (meaning excessively) and "payment" (from Old French *paiment*, meaning the act of paying). The word refers to the act of paying more than what is due or required.
Think of paying 'excessively' ('over-') — that's why overpayment means paying more than necessary.