This. In my experience the “buyer didn’t receive package” claims are usually handled by the seller and the seller is the one who deals with UPS since they are the one that paid UPS. If the seller does nothing to remedy the missing package or refund the money, then the buyer’s next step is to initiate a chargeback with the credit card company. The sellers normally lose that battle. Even if the seller did everything right, if the customer makes a reasonable claim that they didn’t receive their package then the credit card company will side with them.
The seller may be able to initiate a claim with UPS and get their money back if they had insurance on the package, especially if it was something like a signature required package and no signature or an incorrect signature was obtained.