on time good/reliable customer. Customer always pay on time. Is always on time with payments. Not on time. Delinquent/Unreliable Customer. Alternatively you might hear a customer who is always in arrears or a customer who is always late [with payments].

Not very elegant expression but it is often being used : good payer / bad payer

on time simply - a good customer/ reliable customer not on time - poor customer/ unreliable customer something abit more idiomatic would be" getting him to pay is like beating blood out of a stone"