You can leave feedback as usual about the transaction in general but you cannot say for example "Great item but slow & expensive shipping" (referring to the GSP). You cannot make a comment about the cost of the GSP or the length of time it takes to arrive.

But go your hardest if you get the wrong item.....that is nothing to do with the GSP.

What if I make a comment such as 

"The item has one crack and the shipping was slow"?

So basically I can leaving a feedback for both the item and the shipping, I need to let the seller know that buyers are not happy with the shipping service, since it is us the buyer who pay for the shipping, then we have the right to choose which shipping service we use.