Package being held in customs, shipper claims they didn’t even send with Fedex

I have a very strange situation right now. I have a package from Korea being held in customs in Alaska because the shipper did not provide sufficient information for it to clear customs. When the company emailed me my shipping information, I was given a tracking number that starts with UP and ends with KR.

I received notification about the package in my Fedex app. A numerical tracking number is listed, and the UP number is listed as a reference number instead.

I asked the shipper to contact Fedex to provide more information for the package to clear customs, and they said the package was not sent with Fedex and was instead shipped with “EMS Premium (UPS)” and it’s my job to deal with any customs clearance or fees in my country.

Was my package picked up by the wrong shipping service somehow and that’s why it’s being held? And is it up to me to contact FedEx or does the shipper need to do it since they didn’t provide sufficient info? I can tell Fedex what’s in the package but I don’t know if that’ll be official enough, and I’m not sure the best route to contact them about it anyway.

There’s an album signed to my name in this package that won’t be replaceable if it gets lost, so that’s why I am so concerned and want to resolve this as quickly as I can. I’ve just never heard of this happening before.