The shipper needs to stipulate on the commercial invoice what broker will clear the item. If it's you, or your customs broker or whatever.
UPS makes that a very difficult process at the US end to set up though, they like the cash cow of acting as a broker and frieight carrier.
Just as an aside, the rules that folks like UPS play by were established back in the days when railroads were first crossing the continent, they are tilted towards giving the freight carrier all the cards in the deck when things go sour.
Airsoft, where nothing is hurt but feelings.