Some background: In Germany (at least) invoice numbers have to follow certain rules:

1. The have to be ordered
2. They have to be continuous (may not have gaps)

Since a few months they are allowed to contain characters. Some customers want to use that possibility and customers don't know that or are afraid and they insist on digit-only invoice numbers.

Additionally the customers don't want to start them at zero.

Is I can think of many ways to generate such a number I wonder: What's the best way to do this?