A number of years ago, my daughter sent £100 in Next gift vouchers and a cheque to settle her Next account. I actually put them in a sturdy envelope & typed the address to that it was quite clear. I posted them Recorded Delivery, obtained a receipt and also made a note of the serial numbers of the vouchers.
The envelope did not reach its destination. Through the serial numbers on the vouchers, we were able to ascertain that they had been exchanged for goods at a Next store in Manchester.
We were, of course, not able to prove what our envelope contained. I believe my daughter was awarded a book of stamps by the PO.
I rest my case.