Can I send a manilla envelope at the blue postal boxes up the street?

Or would I have to go to the postal office? It will only contain about 1 or 2 pieces of paper. Thank you.Can I send a manilla envelope at the blue postal boxes up the street?
anything under one pound in a manilla envelope or smaller goes in a mail box with regular postageCan I send a manilla envelope at the blue postal boxes up the street?
asdasdasdCan I send a manilla envelope at the blue postal boxes up the street?
as long as it fits and you have the right postage it doesn't matter