Well, the postage stamp was invented in England.
England has a long history and was originally invaded by the Romans who built straight roads so the armies and the mail could get around easily.
Then the Normans under William the Conqueror invaded in 1066. They introduced all the people called Fitz to the country. And so it became a one size Fitz all country.
One of the Conqueror’s sons also ruled England and he was named William Rufus or William the Red, People named William are often called Bill.
And that is why letter boxes are painted red. They are always full of Bills.
Note; No correspondence will be entered into regarding the veracity of this tale.