It’s the fee for them to file the import paperwork for you. Every shipping company charges some amount for it. The customs broker we use at work charges $100 to do it. If you don’t want to pay it you have to call and tell them to hold your shipment at the border until you have it cleared through customs yourself, not a new thing.

I don’t have any sympathy for someone who is clearly aware of how it works and thinks they can just bully their way out of paying charges they’re responsible for. The receiver is presumably also an adult who knows how to drive down to the UPS depot and pick up the package they refused to take delivery of the first time. You’re doing way more than you should to for someone who doesn’t deserve your time.