To be honest I don't know whether the following suggestion will still work as I gave up selling on eBay when they did away with PayPal as the means for sellers to receive payment from buyers, but if things haven't changed too much since I last sold anything on eBay and the following advice has not been rendered obsolete in the meantime then you could try the following in order to get around the problem:
Has the buyer paid yet? If not then revise the existing listing, delete Evri and change it so that Royal Mail is the only postage option available to select when making payment for the item. If, however, the buyer has already selected Evri and has paid for the item then contact the buyer via eBay Messages and state that you can send the item via Royal Mail if he or she wants the item to be posted that way, but that you would have to cance the original sale and relist the item again with Royal Mail as the only postage option so that he or she could purchase it again using his or her preferred courier.
Should your buyer agree to the above then ask him or her to request a cancellation so that you can shut down the first sale. When you receive the buyer's cancellation request accept it and you ought to get your Final Value Fees refunded in full. Once you've done that relist the item again, making sure before you submit the listing that there are no other postage options other than Royal Mail on the listing.
As soon as you have submitted the revised version of the listing send the buyer another e-mail via eBay Messages, saying something along the lines of "I've just relisted the item that you wanted to buy, and have set it up with the courier showing as Royal Mail. In order that you can find the listing faster, run an Advanced Search and, using the Enter Keywords Or Item Number, enter this item listing number [insert new listing number here] to locate the new listing", so that your buyer can quickly locate the listing and purchase the item before another buyer has the chance to do so. However, make sure that you use a tracked postage option, such as Royal Mail Tracked 24, Royal Mail Tracked 48, or, if it is a very expensive and/or valuable item, an enhanced service such as Royal Mail Special Delivery Guaranteed By 1:00pm. That way, once eBay put a trace on the tracking number and can see that the item has been delivered, they shouldn't have any justifiable reason to withhold the payment if the buyer has not opened a case against you, and they ought to release your payment to you without any fuss.
