I believe that a large number--if not the majority--of listings that have the Global Shipping Program applied to them have had it applied automatically by eBay. There's a section in the .com user agreement that states that eBay will sometimes do this for listings that have no international shipping specified and the seller hasn't blocked bids or purchases from users where their primary shipping address is not located in the United States.
I have a feeling that your seller is one of those who hasn't been keeping up with changes to the .com user agreement and has had eBay apply the GSP to that particular listing. They probably didn't have a clue what was going on when they contacted you.
I contend that the GSP would probably be better understood, or at least expectations about its functions better managed, if it had been marketed as a forwarding service, but I guess calling it something like the Forward-Ur-Package-Service (F'UPS) doesn't have a very good ring to it.
Erlanger, Kentucky is the hub for GSP shipments. It's officially referred to as the "Global Shipping Center". The GSP terms and conditions page for buyers actually has quite a wealth of information in it that's also of use to sellers, who don't have anything similar as they're not the ones paying into the program, but it doesn't mention Erlanger by name. The CSR to whom you spoke probably had similar information in front of them.
The GSP does have its place, albeit a very limited one. After doing much homework on the program, I decided I could put up with its quirks and purchased two cell phones that were being forwarded through it. It was a lot less expensive than the overkill tracked shipping methods being used by many other sellers and it arrived relatively quickly. In addition, the customs clearance fees couldn't be beat. However, I haven't purchased anything else where the GSP was being used because it simply didn't make sense to use it, and I think many other Canadian eBay buyers are finding the same thing.
