i too think it's a bit hit n miss.
i take all my post to be weighed, i like to know i have the right postage on it.
but as it's reliant on the person behind the counter to use the plastic 'postal' slot to check the size of the item, i think they should have a rubber stamp to say it's been passed as checked.
my birthday card from my mum came and needed £1.06 paid on it, mum was mortified, it was a shop bought one too!!!!!

chell
x