Tip Option
I use invoicing for both products but also for services. It would be really nice to have a tip feature that I could choose to turn on when I bill for a client session, that would give the client the option of leaving a tip. If as the photographer you don't want to do that, you just wouldn't turn that feature on.
When I used to have clients venmo me, I used to get a tips pretty often. When I invoice through instaproofs, I never get tips.
As a consumer, I always tip especially if the person did a really good job or if I'm really happy with them.
I think it would be a nice feature to have as an option.