Assuming the buyer is domestic, the fact that the listing was eligible for global shipping shouldn’t prevent you from combining shipping.
If the buyer is out of the country the. I don’t think eBay allows sellers to do combined shipping on items that are actually shipping outside the US via global shipping. If that’s the situation you find yourself in, then the best thing to do is cancel the international sales, create a new listing with all the items and have the buyer repurchase. If you do that, eBay will only charge a single “global shipping” price, which will save the buyer a lot of dough.
Hope this helps.
Edit: it looks like you already figured it out!
