The GSP as offered by eBay seems mostly a money making exercise for Pitney-Bowes as far as I could tell, mostly due to the markup on their customs fees as well as their shipping. It does seem to be a desperation only option. As said above, stuff from China seems to pass through with no issue regardless.

I'd suspect if you could find a more economical shipping option (likely slower than the GSP offering) that those could work out a good deal, if the seller was willing to use it.

Yep, GSP is a rip off. I buy a lot of all sorts of bits from the UK. The difference in Old Mate dropping it in the mail for £10 or GSP at £50 for the same item. Out of the US it is absolutely ridiculous! Trying to buy a single page illustration, as 1 x A$(ish) sized sheet and GSP is $76!!!