With Simple Delivery the items have to be bought in one transaction at checkout for them to be combined.
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 More Actions dropdown menu for the items on your orders list.
An alternative would be to send them together but some of the items won't be tracked. You would still receive payment after 14 days but you risk the buyer claiming non-receipt. Unlikely but possible.
You should generate the unused labels to mark the items as dispatched. Afterwards if you paid for the extra postage labels you can cancel them here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
If the buyer paid for them they will be refunded automatically in 30 days.
