I've come across this GSP when trying to purchase mutiple itmes form the same seller and thought it was stupid to. The seller was quite blunt and rude to me too when I questioned the amount ebay wanted to charge me for the two separate shipping rates
He rambled on in an email about how it's out of his control and implied it was something ebay had enforced upon him as a power seller...

I noticed a few months ago (the feature may of been added before this but I don't recall) that ebay actually has an add to cart option now if the seller has multiple items for sale. As someone that has had some experience selling on ebay you have the option (when listing an item) to put in its dimensions, weight and tell the postage calculator to add the weights together if a person buys multiple items so that it automatically calculates the cost of shipping (saving them money). It seems that its smart enough to get the cost directly form the selected carrier based on the numbers you enter. It's really dis-appointing that the GSP doesn't do this too and only calculates individual items making the already expensive exercise of buying from the US even more expensive
Hurry up and fix this ebay...