If you buy an RM label now you'll be paying for it yourself. If you paid for the original label you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
If the buyer paid for it it will be refunded to them automatically in 30 days.
It's better to cancel/refund the transaction with the buyer's agreement (via the dropdown actions menu arrow for the item on your orders list) and relist after opting out of Evri postage, for the buyer to buy and pay again.
You can opt out of using Evri here:
https://www.ebay.co.uk/ship/prf