Global shipping is a different company to Ebay. Ebay allows a buyer to mail to a different address and to combine postage for more than one item. 

However Global shipping takes your registered primary mailing address and  assesses postage based on that address. If you try to change the address the seller cannot change it. Ebay wiull not change it as they say Global shipping is a  separate comoany. I was advised to buy such an item wehn logged in under a diiferent mailing address. This does not work and Global shipping continues to charge based on where usual shipping address.

If they were genuinely interested in doing the best for bsuiness they would assess the postal charges based on where the item is to be mailed. I won't hold my breath as this has been a problem since Global shipping started.