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Some more information!
I made some enquiries of Post Office Counters. Apparently, the reason the machine cannot be used whilst the counter is closed is "in case any customers encounter difficulties" with their transactions. I accept that the machine does not simply dole out stamps, but it also weights items and calculates prices so things might get a bit tricky. I suggested that all that was needed was a big red button which would enable customers to have returned any money deposited should they "encounter difficulties" but was told that could not be done.