They need to open a return case.
Then, if you choose you can offer a partial refund and they will keep the item.
Or, they can post everything back for a full refund and you can try to claim against the courier.
"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)