Several years ago I purchased a Fender Custom Shop bass from Canada. The dealer shipped it UPS. I received a customs bill for around $600 from them on a $3,500 purchase. No details were provided. I still have no idea how they calculated it but the bass was within NAFTA rules at the time so there shouldn't have been a charge at all except for UPS's import fees that were like $50. Took some time dealing with UPS but they eventually waived the fee. I would definitely call them and ask for a detailed invoice and maybe even contact customs to confirm the amount is correct.
I bought a pre CBS Jazz Bass from Canada in the 90s that was shipped and brokered by UPS (to California) and was initially charged import fees but was able to get it reversed once it was clarified that it was produced in the US (despite being bought from Canada). Was told if doing the same in the future to specify that it was "US goods being returned" (to the US rather than a new purchase return/exchange). I did buy another, had the forms so marked and had no fees charged IIRC (maybe just UPS's flat brokerage fee). Obviously does not apply in the OP's case though.