When I did a bit of reading up on the GSP a little while back I learned a few things. Main one being that large, heavy items are far better being sent that way because it's far cheaper. Smaller items are usually ridiculously expensive. However, if the seller puts the weight of the item in, it spits out a more reasonable price.

For example, Seller A is selling a spanner that weighs 50g and doesn't insert the weight into the postage section, so the international buyer might see a postage quote of $45. Seller B is selling the same spanner but does put the weight of 50g in, so the international buyer will see a much lower postage quote of say $15.

Problem is, a lot of sellers don't know to do that. There are also quite a number of sellers who don't even know they are using the GSP, so would have no reason to do it.