[mothered]

if you know your post office well, you can come in with say 4 big ish boxes to send in a big bag, scan them all and get the receipts (including your return laptop for example)  put them back into the bag every time you scan.. usually they will point to a window for you to put your boxes into..because your boxes are big they will ask to leave under it or round the corner etc.. then you simply walk over to that window and only take out 3 of your boxes and place those under the counter

This has worked with a number of different stores for me with obvious easy refunds after persisting it was lost in the returns process.

Wouldn't the carrier scan the packages at the collection point, and deem that only 3 boxes were collected and not 4?

The reason i tried this was for this exact reason.. i returned a Hermes package and it never even got scanned.. i'm assuming the driver stole it.. and no one cared. I got a refund anyway because i scanned it in.
With 'RoyalMail 24/48hr signed for' service their tracking  also only tracks the scanning of the item in and the signature on arrival. So I guess Hermes and Royal Mail are both good candidates for this..

This Is very Informative, carrier-wise.

Thanks for sharing your experience.