this is an example of one of my invoice templates on paypal. this is about as much detail as you should ever include on an invoice, and you can put less if you choose to, down to just the price total and nothing else

i'll add some specifics, like a character's name, if i feel it helps the client's comfort level or if there's multiple characters i'm charging for, to keep an itemized list of what's being paid for, but that's it