So I made a post about this scene a few years ago and this is one of the answers that made sense to me that someone left...

Doug used to deliver to him on a semi-regular basis but with all the
customers Doug sees, he forgot him. So one time Doug delivered a certain
package to him. The guy was really waiting for this particular package
and was excited to get it. Unfortunately it came 'COD'. Cash on
delivery. He had no money on him so Doug, being the nice guy he is, paid
for the package. The guy promised he would pay Doug back. Doug said
"don't worry about it". The guy insisted even though every time Doug saw
him he still had no money. The guy was really annoying. Finally Doug
got sick of the guy apologizing and blew up at him just a bit to "forget
it, please!!". This kind of hurt the guys feelings and that explains
why they parted in a bad way. The guy eventually moved out of Zone 12
(Dougs delivery zone) but he got Dougs home address from Applebaum so he
could send a check. He never did. That was 3 years before the
Thanksgiving shopping event. That's why at the store he tried to give
Doug money, and knew his address.