First, just to correct the other persons answer, ebay does offer combined shipping.
Okay now don't panic! its not so hard as it seems. when you clicked the invoice button, you asked for a correct price from the seller, because eBay didn't automatically merge the postage costs. NOTHING on eBay is posted UNTIL it has been paid for. some people buy stuff and never pay for it, they just leave it sit there. you could either write to the seller and ask nicely if you could cancel the item you didn't want (as it was a mistake or something) or you could pay for the thing you want, and tell them you'll pay for the other item later, and just never pay for it. it will be cancelled after a while anyway, and nobody loses because they still have to item to resell.
It really isn't big deal, so don't get worked up :)
Experienced ebay shopper.