With Simple Delivery if the buyer buys the items all at the same time, putting each in their basket (on a PC, not the app) they will be automatically combined. If they are bought separately they will not be combined but you can just send them together anyway although some will be untracked.
Mark dispatched by opening the unused labels (Get prepaid label > Get prepaid label and QR code). They will remain unused and if you paid for the extra labels (had free postage on your listings) 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. The payments for the untracked items will be released in 14 days.
The alternative would be to cancel/refund the transactions with the buyer's agreement and relist with items on one listing and a single postage cost for the buyer to buy and pay again.
For future sales you could put a note on your listings that any buyers wanting to buy more that one lot should let you know first. Then you can relist those items into one listing with one postage cost.