You don't have to buy your postage from Ebay.
Take your parcel to the Post Office and post from there buying postage at the counter.
You may have undercharged the buyer. 87p is the cost for a letter which has to be 5mm or smaller. I am not sure a CD in a padded envelope will go as a letter.
"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)