I sent a message to customer service and finally got a reply this morning. It could be of use to people and also to US sellers that don't know. This what they wrote:

"Thank you for your interest and questions about the global shipping program. The seller does have a choice in whether or not he would like to be involved in the program. If this is something that you guys wish to go outside of the program for, please click on the shopping cart in the top right hand corner of ebay and then click on request total from seller. This will allow him to send you his own invoice and not have it go through the global shipping program.

If the seller still wants to go through the global shipping program, the only other option you have is to ask the seller to cancel all of the listings, list them all together (instead of separately) and then you purchase one new item number. This would allow for you to get a better rate on shipping. I apologize that the shipping costs are so high, we typically suggest that a seller not use the program if the item is less than 1 lbs and under $30."

That was probably more useful than the reply from them that was posted above. This wouldn't have worked with my $100 seller as he wasn't interested in going to the post office, but for sellers that are willing and don't know about this, it could get both buyers and sellers out of a pickle!