So, over at that wonderful LJ community known as Artists Beware there's presently a brief "advice for artists" type post about using PayPal's invoice system. From what I'm seeing it's a good handy system, especially for those artists who work in currencies other than the "commissioner default" of USD. It also lets you break down things like different portions of your commission fees, like showing how a picture starts from the single-character full-color, adds a second character, then a background, etc. You can provide discounts with an item having a negative price (example of a "preferred subject matter") or give it a bulk discount by percentage or dollar.

You can include, if it's under a thousand characters, some light TOS. If you like, you can leave it as payment due on receipt, set standard NET X terms, or set a specific date.

The trick is you need to have the commissioner's e-mail address. I've got my "big three" attached, that is to say my primary, gmail, and hotmail accounts. I'd actually done this in case someone would be more comfortable with a payment coming from the address they talk to, but it should be helpful for these invoices too.