Been using this company for a number of years. Last trip they left an invoice and I paid it in full.
2 months later they called and said THEY had made a mistake and that I had a balance due and they would be turning it over to COLLECTIONS if I didn't comply. I do not appreciate threats, especially those resulting from your mistake. I am posting my experience in multiple locations and relating their customer relations methodologies in my meetings at work. Simply asking would have worked much better.
If you use them, I would suggest you have them write Paid in Full on your invoice.