Things happen, just like your household bills, on rare occasion we slip. We usually start with a "did we overlook something" statement. We never demand, only inquire to resolve. We then follow up with a phone call and a physical invoice being sent, along with an impassioned plea for them to take care of the bill we agreed upon. 

Finally, we suggest return of our product if payment is not an option. If we still receive no joy, we write it off as loss, in our view, one loss, in every 1000 is  not the end of the world