If you can buy by adding the items to your cart then I don't see the problem.  It might be an extra step but does it matter?

It's usually if a seller hasn't created combined postage within the eBay listings so you would need to wait until a listing is set up with the items you want. However using the cart will do a combined postage and therefore can be purchased n paid for immediately.

I've had it happen a couple of times on single item purcahses so not necessarily on multiple items,but seems to work if you add them to cart