Have they refunded you without needing you to return the item?
Phone cases are usually inexpensive so a seller might refund you without providing a label.
Of course, if you have not had your money back, you should, as advised, get Ebay to step in.
They will provide a label on the seller's behalf or just give you your money back.
"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)