In HVAC trade, a lot of money arrives late and on paper: post-dated cheques from contractors, instalment plans for retail customers.
A cheque is a state, not a payment
A cheque is received, then it clears or it bounces. Until it clears, the customer still owes you. Symtex records it as such, and a bounced cheque puts the invoice back to unpaid.
Reminders, once each
Cheque dates and instalment due dates appear as reminders, and are never announced twice. The Cheques & instalments screen shows what is due.
Credit is cash-only by default
A customer's credit limit starts at zero, so nobody takes goods unpaid unless you decide they can.