In the 90s I had this problem with Sun Microsystems (it's called "aging payables"). If you give a discount for prompt payment they take the discount and pay when they want. If you charge a penalty they pay when they want and ignore the penalty.

I needed to make payroll so drove over to HQ to try to get payment. They wouldn't even let me in past the lobby. Finally I just asked everyone who walked in, "Oh, are you here to try to get them to pay their overdue bill as well?"

Eventually a confused looking guy came out and handed me a check.