Refers to charging more than the proper amount, especially in a commercial context. Can imply a deliberate or accidental extra charge.
This shop has a reputation for frequent overcharging.
When I checked the receipt, there was an overcharge.
Compound of 付け (tsuke, 'bill' or 'account') and 掛け (kake, 'credit' or 'charge'), referring to an amount added to a bill. The exact historical derivation is uncertain.