I'm not in the photo business but did run a contracting firm for 20 years. I usually had net 30 days on my invoices but that was really something to fill the page. No one pays interest on a $750 invoice or $750,000 invoice unless you sue them which you're not going to do. It's not going to force them to hurry up the payment.
It's really the people you're dealing with. If it was a larger company with an accounting department who regularly only do payments, the invoice would have to be approved by the originator who requested the work that I sent it too first before it went to accounting. So the key is to call that person you sent the invoice too and ask him if he processed it and if not would he do so. If he doesn;t have it handy, ask him if you could email one right now so he can process? In the old days, I would fax or mail, but the trick is to get it done while he's hot to help you (if he is.) If you suspect he's got a problem with your work, be upfront about and clear up the problem. If he actually approved it and it went to accounting, then its how fast that company processes checks and gets them signed.
If my work was for a small company were the buyer just handed it to his secretary to issue the check, I would follow the same procedure. The key is personal relations and persistence, but with a smile. And call. No one pays attention to the terms. So you have to push, but politely.