So get this -- I received a bill from FedEx on Monday for $43 and change for a bunch of surcharges including shipping and handling. I was pissed so I immediately called the number in the envelope (FedEx Revenue Services at 1-800-622-1147). I explained my situation to the lady I spoke with. I told her I sent back some defective merchandise using a pre-paid label from the manufacturer. She told me the vendor on the account refused to pay the surcharges because they didn't have proof they were supposed to receive my package, therefore they invoiced me. After I hung up with FedEx I immediately contacted the person I spoke to at Warner and left a message. She called me back later and I explained what FedEx told me. She was pretty angry too, (1) at the amount they charged me, and (2) for the fact this happened.
I scanned a copy of the pre-paid label from Warner as well as a copy of the FedEx bill and e-mailed them to her as .pdfs. She said I was not responsible for the bill and it would be taken care of. She also said if I received anything else from FedEx to call her immediately.
The bill is due on February 16 and I would rather get it taken care of sooner rather than later. I DO NOT want to deal with a collection agency.

I don't want any headaches.
Anything else I should do to protect myself? Basically I don't want FedEx coming after me.