<<The GSP disallows a seller from combining items making the shipping very expensive.>>
Well, yes and no. If the seller offers combined shipping and the buyer puts all the desired items into the shopping cart, there should be a discounted amount. It may work for the GSP portion even if the seller has not set up discounted rates for multiple buys. It's still expensive, and if the items were low-priced to begin with, and the total will be more than $20 Canadian, (about $14 US) then the whole package will have import charges added.
So in a way, that might nullify any 'discount' on the shipping.
Better to ask the seller to ship the items directly with USPS First Class, in my opinion. 