You call out for a Pizza. They deliver it 30 minutes later. They hand you the pizza; you hand the delivery person the money plus a tip.
In a small town, J Furniture Company will deliver large furniture. They expect 75% paid in store, with 25% paid on delivery (plus delivery fee and tip to the workers).
You contract to get new windows installed in your living room. The contractor requires 50% down plus cost of the windows, and the remainder when he installs them.
You've gotten arrested. The Judge orders you to pay a $300 dollar fine. You pay--you get out of jail. You don't pay--you stay in jail. Paying the fine is system's "cash on delivery" required for your freedom.
You call a plumber to unclog your drains. He expects "cash (or check) on delivery" of his services.