Nobody's ever had a usable answer to it; I throw it out here in case someone reading this has one.
Why do companies send you bills for a specific amount, for which their known policy is "we do not accept partial payments, payment is due in full only".....
.....and then put a box on the payment slip for you to fill in that says "Here' s how much I'm sending."
"Hey! Ya owe us $128.37! We demand that you send us $128.37! No more! No less!"
"So! How much ya sending us, huh?"
Only company I know of that doesn't do that is American Express. They presume their customers are bright enough to understand the rules......