I sent four items I sold using one postage label with the new delivery method. I can now not add the tracking for three of the four items or mark them as dispatched.
Is there any way of resolving this so they don't show as late?
You have to generate the unused labels to mark the items as dispatched. Afterwards if you paid for those 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.