Two comments from me.
Firstly, this is not Ebay's fault. The postman muddled the labels and this is not an issue Ebay can help with. You will have to sort this out with your buyers.
Secondly, if you were properly registered as a business seller, you would not have to use Simple Delivery.
"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)