Does the seller offer a combined shipping rate? Sometimes it is applied automatically at the checkout, and sometimes you have to wait for an invoice. What does the listing say?
Usually it's as easy as clicking to buy the items, and then the instant the seller sends an invoice the adjusted shipping rate appears in your Purchase History. Click 'Pay Now' and pay for all the items at once.
If you are buying from different sellers but in the same currency (no combined rate, though, obviously) you should be able to pay it all at once.
The exception is if the seller requires immediate payment. On the Canadian ebay you have to pay one at a time, or you can go to ebay.com and use the shopping cart feature. Putting items in a shopping cart does not guarantee you get them, not until you actually pay, but you can add numerous 'immediate payment' items and put it all on one tab that way.
