You can make a special listing for them, putting all the items into the listing with the correct combined postage.  You can use "special listing for whoever" in the heading.

I've done this for buyers & it's worked OK. 

I've also just changed the postage on the listings so it equals the combined total & request the buyer to buy ASAP so it's not sitting there for anyone else.

If buyer buys items separately without contacting me first,  I give them the combined postage rate & issue a refund for the excess postage, taking out Ebay fees first.

I've never had a problem with any of these methods.

 Hope this helps you.