On The Post Office

Forget physicists mucking about in universities, Forget classified military projects, Forget big corporations moving into experimental science.
The post office has already beaten the laws of time and space.

I had several boxes of letters taking up my entire boot, but did that phase the mailbox? nuh-uh, it took those letters with room for more. The only explanation that makes sense is that they are bigger on the inside than the outside 'cause those boxes certainly aren't bigger than my boot.
Maybe it's a variant of L-Space whereby the letters in the box warp space just enough to fit a few more in, which must make post office workers well trained so the mail doesn't explode outwards when the box is opened.

The post office already has Yesterday Mail, they just don't want you to know about it.