Send them a polite message explaining the issue and would they kindly accept a cancellation request this will fully refund them and you can relist the item again on eBay for them to buy this time with a Royal Mail postage.
Hopefully, they'll agree, you then cancel citing Buyer requests to cancel, so it doesn't impact your seller performance.
Relist, untick the Evri postage, you will find the RM postage suddenly increases! Simple Delivery deems the price to be more as it gives a buyer less choice of service ! 🤔
@buffyphipps