There's always a way - Pitney Bowes has corporate offices - call,
explain the situation, get a real person to look into it for you.
Threaten legal action against them if you feel that's the only way to
get them to listen to you. There are all _sorts_ of possible
explanations, e.g., maybe the shipping program has decided not to handle
guitars except from specific manufacturers who've signed off on not
sending things with banned materials - and maybe ebay doesn't know this,
and maybe your seller doesn't know this. I'm just inventing things now,
but it's perfectly possible, and until you have an explanation, you just
don't know where the problem lies and where any fault may lie as well.
http://www.pb.com/contact-us should get you started.
NB: I have no experience with the program you're talking about, or
Pitney Bowes, or the seller, or you. I'm just saying that, if you poke
around enough, you will find someone willing to answer your questions or
you'll find a lawyer willing to take your case.
-S-