The seller does not make any money from the GSP. 

The service is run by logistics company Pitney Bowes....the seller sends the goods to PB who then repack it and process the shipping documents before handing the parcel to the overseas courier company.

It has always been a very expensive service for saller and lighter ites although it is more cost effective for larger and heavier items.

It is also a rather slow service so be prepared to wait for your items.

The usual advice given on the boards is to avoid sellers using the GSP.