The usual suggested way round this is to cancel sales with buyers agreement, then list all together to be bought again and shipping as one item.
Damn nuisance but in addition to saving on postage, the buyers fee will just be one 10p plus the % calculated on total value.
Saves a small amount for buyer.
I am convinced that ebay has no intention of making combined postage possible, as they make more money by not allowing it.
The risk with this is that buyer may not buy again, or not understanding the reason, thinks there is something fishy going on.
