If you've already paid for a number of items you ordered from the same seller, and the packages haven't been marked as sent, the seller may still be able to combine postage if you ask them.
How to combine postage
Many sellers automatically combine postage if you're buying multiple items from them. All you need to do is add the items you want to buy to your cart and you'll see the combined postage total at checkout.
If the postage section of the listing doesn't specify postage costs for additional items, you'll need to ask the seller before you pay. If the seller agrees to send the items in one package, they'll send you a combined invoice detailing all the items and the new cost of postage.