There are two ways you can combine postage - one, as described above, is to send an amended invoice once the buyer has finished their purchases, in which case you select Send Invoice from one of the items in your sold list, adjust the postage price manually (don't click Recalculate, as that just reverts postage to original cost), then click send (or preview the invoice, and if everything is ok, send).
The other way involves setting up automatic discounts which are applied when the buyer proceeds through checkout (if they pay for all items in one transaction). This works best if your items will have a relatively consistent cost for multiple purchases (for example, with most the items I sell several can go in one package at no additional cost, so I have set up automatic discounts for all additional items to have no extra postage charged).
Other automatic postage discounts include adding a set amount per additional item (eg highest post charged for the first item, then $1 per additional item), a percentage discount of the P&H for each additional item ( highest post charged for the first item, then your set percentage off the stated P&H on additional items, so if a buyer purchases 3 items at $10, $8 and $6 post, the full $10 is charged, then they'll get the discount off the $8 & $6). You can also use promotional postage to have a maximum postage amount (in which case P&H charges will accumulate until the max cost is reached), or to make it so that if a buyer purchases a set value or certain number of goods, postage will be X amount, including free (eg buy 10 items, only pay X post, or buy $100 worth of items, only pay X post).
There is an option to combine calculated P&H automatically (where the P&H is not entered by the seller, but calculated based on the weight of the item), just keep in mind that if parcels reach a certain size, AP charge by cubic weight not dead (actual) weight, so if your items are relatively large, or putting several together will take it over the allowed size, this option could cost you a bit with undercalculated P&H.
If you want more detailed instructions for creating automatic postage discounts, just let me know and I'll put some up when I can.
