With Simple Delivery the items have to be bought in one transaction at checkout to receive a single postage label for the items to be combined. Even then there are often problems.
If you don't want to send the lots separately it's best to cancel/refund them and relist in one listing with one postage for the buyer to buy again. You do that from the dropdown menu for the items on your orders list.
An alternative would be to send them together anyway. Open the unused labels to mark the items as dispatched then change the tracking at View Order Details > Postage > Track Package > dropdown arrow. That has to be done on a desktop browser.
If the buyer paid for the unused labels the cost will be automatically refunded to them in 30 days.