Re: Mailbox on concrete
Sweetpeas won’t be their best in a container because they want deep cool soil. You will probably want a minimum of 18″ of soil.
I would also be concerned about growing them on concrete, as the concrete may hold too much heat if part of it is exposed to the sun. If you decide to go ahead, I would plant just one or two plants as sweet peas branch heavily. For supports use some garden twine tied on one end to the mailbox and the other to some earth staples pushed into the soil. I have successfully grown Thunbergia alata (black-eyed susan vine) this way around a mailbox, which might be a good alternative for you.