Hi WillGoss,
I don't have a way to combine shipping on Amazon. When I started shipping with them I used to refund the excess, but Amazon counted this as me rectifying a mistake I'd made and sent me warnings about correcting my business model. (This somehow cut into their profits.) The only encouragement I can offer is that a book I'd normally sell for $18 at a model convention is now $13 with the equivalent of "free shipping." Wish I could do better on this...
Cheers,
Dana