I thought apple used courier?
I Take it you did not use the advance replacement service. (where they send by courier the replacement you then box it up and call the courier company to collect and return costs about £30.00 but secure)
How far is your nearest apple store. As that is the recomended route for all apple repairs
If you sent first class it is only insured for something like 30-40 pounds.
This has been discussed in relation to returning to o2 as well, best to get propf of posting or paying the small ammount to send recorded or the £5 or so for special delivery (it is a £500 bit of kit)
iPhone 11 Pro 256gb on unlimited data
iPad Pro 12.9” 2020 256gb refresh o2 family discount
Apple Watch series 4
My first mobile was in 1995 a CM-R111 from sony on Cellnet.
Wincanton South Somerset (Full 4g 3G 2g indoor coverage) Remember we are all customers here not customer services