The seller should be asking the question, not you. Not that you couldn't but you do not need to explain to the seller what the seller could easily find out for themselves by reading the GSP policy and doing a little research.
From what I've read, combined shipping cannot be done through the GSP.
Here are questions and answers from ebay.com on combined shipping and the GSP:
"ilovemysassycat, asksMarch 12, 2014
how can i combine shipping on items that are sent through global shippping program?
My buyer is frustrated because I can't send a total invoice with reduced combined shipping.
lucy*davies replied:March 12, 2014
The only way it can be done is by using the shopping cart facility.
Best bet is to ask the buyer tocancel all sales and you then relist as a combined BIN and send by normal USPS mail.
Then opt out of the GSP. your current low value items would cost any overseas buyer a fortune. Set your ship to destination and hit get rates to see what your buyer sees."
"zo6driver, asks January 08, 2014
eBay offers no combined shipping for global shipping and buyers are not wanting 5 separate parcels and increased cost.I am about to lose anothe large quantity sale due to this.
silverstrad replied:January 08, 2014
GSP does not allow purchases to be combined.
Each parcel needs to be sent separately and paid for separately, including the shipping and "import" fees.
It is of course your choice to use GSP.. or not.
But that decision needs to be made before listing."
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.
