How do I locate a lost iPhone in transit during an exchange return?

I exchanged my iPhone in June and returned the old iPhone via the return box and label. Somewhere between posting and receiving the phone has been lost although it is verified as “in transit”. Unfortunately

I can’t find the receipt of postage and I’m still being charged for two phones. One of which I’m clearly not using. Does any know where the iPhones go to be assessed for value? I’ve got a receipt that goes to Coventry UK but I don’t know if it’s the iPhone package.

[Re-Titled by Moderator]

Original Title: Lost in transit