It's usually to do with Evri because they charge more for the channel islands, whereas Royal Mail charge the same as UK.   This causes problems with the postage.

The best thing to do is cancel the transaction with the buyer's agreement (via the dropdown arrow for the item on your orders page) and relist after removing Evri as an option here:
https://www.ebay.co.uk/ship/prf
Then the buyer can buy again using Royal Mail.