20 cms is about 200 mms, nearly the size of the length of an A4 piece of paper; you mean 20 mms. Even that seems too thick for a letter, I've come across this topic trying to buy thin cardboard mailers A4 size, I am doubting they would pass a letter rate standard however, most come in at 16 mms (too thick) Australia Post asserts it is 5 mms. I think I will stick to large envelopes with a cut board inserted.
