Contact the seller, before bidding. Be precise about what you will be buying. Postage and shipping are based on dimensions and weight not value.
If your seller is in the USA and uses GSP, this is even more important. The easiest way for the seller to combine shipping is to make a private listing for you of the items you are buying. Yeah, really.
With a private listing (suggest he uses your eBay ID as the title) he can ship without GSP, or if GSP is still a good idea, he can avoid the program double charging you when you buy multiple items.