Do you have proof of the item reaching them such as courier tracking? Do you use signature required delivery? Did someone sign for the item and/or what evidence can you offer in this regard? If the signature required protocol did not occur due to courier failing to obtain this then it is now the courier company's problem. Lay a damages claim and use the proceeds to (partially) cover the cost of a replacement item.
If you don't use a signature required delivery method, regard this as a wake up call to do so in future.
If you don't use a signature required delivery method, regard this as a wake up call to do so in future.