Dont know much about ebay and its processes but send ebay all the details and how the royal mail mucked up keep a copy of the completed form and any refrence numbers etc from the form or quoted to you.
Ensure Ebay have all the details sent to them bombard them with details on how its not your fault the royal mail mucked up.
Never let anyone disrespect you and if they do make sur eyou set the record straight asap.
This complaint form can you download it off the web? You may wait a while for the post to get though at this time of year.