Has the buyer been refunded? This needs to be done first.
Ask them to open a case for item not as described , accept the case within 3 days and refund through those case details, that way your seller performance is not affected.
Did you buy the Evri postage through eBay? Then, this will show you how to proceed for your refund.
https://support-ebay.packlink.com/hc/en-gb/articles/360014270619-EasyTicket
@michellecars1