"Talk to customer Friday, don't mention payment, talk about photos, normal conversation
Remind customer Saturday the remaining balance is due and I need to ship Monday to get it there on time."

In addition, they didn't mention this to you until Saturday? This entire thing seems fishy to me. Even with what Kotacub said.

Aside from adding this to your TOS, I'd set a deadline for when this thing can be paid off. I'm not sure how to reinforce said deadline though. :/