I suppose it is understandable that some sub-postmasters are unsure about restrictions which apply to methods of postage with which they are unfamiliar because they don't sell them.
Post and Go Labels have a 'use by date' which should be the day after purchase:
But these can be applied and posted anywhere: they do not have to be posted in the Post & Go box, they do not have to be posted at the same post office, nor even in the same town. They can be used the next day hundreds of miles away, and handed over the counter.
Post and Go Stamps, known to all but Royal Mail as Faststamps are valid for all time, anywhere, for the current value of the service shown, in the same way that 2nd & 1st class stamps, and NVI airmail stamps, are.
To help collectors using old Frama labels, I'll add that these are also valid, still, for the price shown on them, in the same way as old (decimal) stamps are - and ½p values are also valid if used in even numbers to make up whole pence:





