If you want to change postage it's best to cancel/refund the transaction with the buyer's agreement (via the dropdown menu arrow for the item on your orders list) and relist with correct postage for the buyer to buy and pay again.
If you've already bought different postage you can't get that cost back from ebay. But if you paid for the original label (would have had free postage on your listing) you can cancel it for a refund 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.
You can opt out of using Evri here:
https://www.ebay.co.uk/ship/prf
