A couple of other members have mentioned this in the forum.
I am sure you know this is not an Ebay issue and you need to speak to Royal Mail.
They do have a customer services number although, in my experience, they can take a very long time to answer.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)