A professionally worded late payment letter can prompt a customer to take swift action to pay the Invoice. You can use a First payment reminder letter once the payment becomes overdue. If the debt remains outstanding after the first letter you should send the Second payment reminder letter when you think it's reasonable, considering the number of days that you have given the debtor in the first letter to repay the debt. If this fails to have the desired effect, you may want to consider a Letter before action to indicate that you are prepared to commence legal proceedings for collection and enforcement of the debt.