The problem
Billing for product and services should be simple. There are no end of packages that can achieve this available off the shelf. But there is nothing more satisfying than having your own billing system written to your own specifications and formatting everything the way you want it.
We had this, for a while, using Lotus 1-2-3 and some macros. As a system it worked, but that was all you could say for it. Billing took three days (yes, that's days) for a dozen consultants and that assumed that they had filled in their time sheets properly and hadn't "damaged" the macros.
This could not go on!
Well, we outsourced the development of a custom billing system and, to give them their due, they produces a custom billing system. The fact that it was unusable was irrelevant, because it did bill customers.
Merry Christmas
Fed-up with the ear-ache I was getting from my boss, I cancelled Christmas one year and set about producing Billing for Dummies. Four weeks later, I implemented a wizard based billing system that was compatible with the outputs of the existing time sheets. Billing time dropped to 1 day, most of which was validating the time sheets.
I then got ear-ache off the consultants about the tim sheets, so I produced a complete replacement time sheet program for them that built in the validation we were doing at billing time. Billing now takes around 20 minutes.
I have the undying gratitude of the bosses; always worth having.