Very thought-provoking, thank you.
Much support and no issue at all...except for a very point. You wrote: "Okay, if, for example, the seller hands over the damaged package at a quiet village Post Office, the nice man or lady at the counter might remember to point it out when the postie comes to collect the mail*, but what about a busy Post Office taking hundreds of items a day,"
Many village post office will be quiet or seem to be. But in my own area and one or two areas I know fairly well at the other end of th country, these post offices are effectively permanently under-staffed, as it's usually only one person handling the tills for post office and for the shop, running the shop e.g. shelf-stacking/receiving & unpacking supplies, if it's a garage too they're receiving fuel tankers, maybe being a drop-off point for courier parcels, and the place may well be part-time. So queues can be long and the place really busy, even if not by a simple numerical comparison with urban post offices, or rather they're extremely busy at certain times - like London buses, nothing for a while then everything. Queues can lengthen (often waiting for them to open) and when people choose them to avoid the long queues in urban post offices in the run up to Christmas. Such queues are testament to customer determination in places where public transport is rare or non-existent (e.g. no public transport west and north-west of me for 70 miles).
As rural post offices tend to be physically small, it doesn't take long for queues to extend out of the door, and because with Royal Mail cuts there aren't enough post offices as are needed. Anywhere (rural or urban) you have to travel a long way to a post office, you try to save up items to post or other errands to deal with in one go, to save on the number of trips, so each customer's turn takes time, lengthening the queue.
Same was true of my local Lloyds Bank, long-since closed, was open 2 hours two afternoons a week but should've been open longer as queues routinely spilled into the street, so it was far busier than an urban branch.
