When you ship UPS to another country, UPS often charges a "brokerage" fee. This is a UPS fee that goes in their pockets not to the government for duties or owed customs. It is an additional fee because they "escorted" it through customs. I suspect that it is this fee that UPS wants from you, not government duties, customs or taxes.
I have been stung by this a few times as a buyer and the fee starts at 25$ and goes up. The first time the seller and I did not know about this. Cost me 100$ at the door. The second time the seller changed the shipping method after the sell.
Did the buyer know you were shipping UPS? Did the buyer and you know and discuss this brokerage fee? If it was a surprise to your buyer, I can see why he refused to pay it. I suspect that If UPS does not get this fee, they will not return your package. You need to ask UPS exactly what fee they are trying to collect from you. I wonder if this falls under duties or is classified as a hidden shipping charge. I don't know the answer. I just avoid any sellers who plan to ship UPS to my country (Canada).
