I'm calling you about a credit purchase you made.............(when)..................We sent you invoice number........dated.......for the amount of Your payment was due on.....( date )....... according to our payment terms. But, until today, we haven't yet received your payment.
Requesting a prompt payment
You're our valued customer and we'd like to maintain a long - term relationship with your company, but we strongly request you to make the payment within ..... (days/weeks)......