Go to the Resolution Centre at the bottom of this page and open an Item Not As Described dispute.
In the dispute, say that this was purchased through the GSP.
You should be refunded promptly and you will not be required to return the undamaged glass.
While the seller is correct that damage in shipping is covered by GSP not by the seller, she should have packed the glassware better in the first place.
Unless the parcel was marked that it was opened and repacked by the GSP itself. They are a hamhanded bunch.
You may wish to reflect this in your feedback and Detailed Seller Ratings.