Unfortunately I don't think the seller actually
can give you a refund...
You see, when a seller uses GSP they relinquish
all control they have over shipping fees to ebay. Ebay is the one who decides the cost of shipping to other countries and the seller won't actually know how much shipping ebay is charging their buyer unless:
a) they look at their listing hosted on another country's version of ebay
or
b) it can be seen after the item has already been purchased on the 'print shipping label' screen.
In addition when you, as a buyer, pay the shipping costs none of that money goes to the seller. It goes directly to ebay so ebay can pay third party people to deliver your item to you. All the seller gets is whatever price they put on the listing for shipping within their own country because all they have to do is send the item to the GSP centre in their own country (and subsequently do not have to fill out a customs form - which is actually one of the main reasons why people use GSP) and the people at the GSP centre take over from there.
i.e. If the seller put that it costs $3 to send a package to someone within their own country, then ebay would give the seller $3 for shipping fees and keep the rest to pay third party people.
The seller wasn't kidding when they said they couldn't send an invoice for combined postage - ebay won't let them because the shipping money isn't theirs to combine.
In fact, if a buyer has purchased multiple items from a seller using GSP, the seller isn't actually allowed to send all of the items in one package unless they're all paid for in one transaction (so there is only
one resulting reference number). If they're paid for in different transactions then each item would have its own reference number and as it states here(
http://sellercentre.ebay.co.uk/using-global-shipping-programme note: this is the UK info page) the rule is one reference number per package.
But, from what you describe, it sounds like you
did pay for all three items in one transaction considering they were all send in the same box. In which case, the seller can possibly refund a small amount of the postage fee if you over paid for shipping within the sellers own country. (i.e. if the seller put a charge of $3 on the listing of the item for shipping fees within their own country, and ebay gave them $9 for shipping because you bought three items, then they may be able to refund you $6)
However
ebay is in charge of the rest of the shipping fee and while GSP lets you combine several items under one reference number, it doesn't seem to stretch as far as shipping costs.
Actually, if you click on that link, scroll down to FAQ's, and look under 'Payments and fees', you'll see this:
Can I offer combined postage discounts?At this time, seller-specified combined postage discounts aren’t supported by the Global Shipping Programme. However, a combined postage discount for the international leg of the delivery will automatically be applied during checkout when the buyer places all items in their shopping basket prior to making a purchase.
Can I use “Send Invoice” or “Request Total” for Global Shipping Programme orders?No, "Send invoice" and/or "Request total" will not be available for Global Shipping Programme orders.
How are postage charges calculated for the international leg?The international postage charge your buyer sees is derived from a number of factors, including the buyer's location, package weight and dimensions. A network of postage carriers will be used to provide fast, reliable and trackable services. Carriers will be selected depending on their suitability for sending your item internationally. For more details, see the Global Shipping Programme terms and conditions for sellers.
And as far as evidence of how much it cost the send the items, I'm not sure you ebay will give that to you... Sorry.