Today I was invited to call at Carnforth Postal Sorting Office to retrieve a 'package' because insufficient postage had been paid.

I handed over £1.16 (includes £1 handling fee) to retrieve an ordinary looking brown envelope approx A4. Inside was a simple A5 flyer invite to the preview of Bay Potters Christmas Exhibition.
It didnt look over weight or over sized, but when I challenged the staff she demonstrated that it was about a centimetre too wide for their authordox standard letter slit.
All the mailout that went via Carnforth was treated the same - there were lots and lots of them, all would have had a card through their letter box saying there was a package for them, collectable on payment of £1.16.
What hacks me off is I cant even go to the exhibition.
Moral:
However good the deal on envelopes seems, check the size so you entire mailing list does not have to cough up £1.16 each.