There are many reasons why you can't combine -

You have Immediate Payment set;

The items have different postage methods;

Any of the items have an active seller promotion;

Probably others.

If the buyer is okay with it, I set up a specific listing for the selected items. Failing that, the buyer can go ahead with the purchase, then you refund the excess.

eBay no longer retain fees for partial refunds, so you won't lose out. Just a bit more bookkeeping.