What happens to non-delivered items?
If an item is undelivered then it would likely be Returned to Sender (RTS).
There are also multiple reasons an item might be returned to sender such as; incorrect delivery address, recipient refuses a consignment or the parcel is damaged.
Depending on the original courier service chosen for the order there might be additional shipping charges for returning to sender (which would be based on the original shipping charge). See Courier Surcharges and Fees
The person who has paid for the original freight charges would be responsible for payment of these additional fees.