I just received a letter from my ISP (dated 5th of February), stating I had to pay my bill. The letter was really unfriendly with threats of knocking me out of their systems. So I checked my bank statement to find out if I missed something, but I had paid properly on January 31. So I mailed the people at firstname.lastname@example.org to inform their of their rather untimely and incorrect letter. And suggested them to bill their three different bills in one time, using a direct debit.
Which goes to show that billing is a profession in itself. See also the oration of George Huitema (here available in Dutch) and the website of the Global Billing Association.