This idea originally comes from the NUMICON Math system, but I have found it very useful for many topics. Also, I made my own mailbox and didn't need to buy one.
First of all, you need to make a folding page as a mailbox, with a slit that you can put mail through. Make it look as much as your mailbox as possible.
I highly recommend laminating the sheet.
(To help the "box" stand upright, I put a small container on the inside bottom, which will be "my" side when in use.)
Then prepare questions and answers. This can be arithmetic problems / a picture-written word / any written question and answer. Cut them out (laminate??), and give him the answers (or the question). You take the second half of the cards.
You slip a card through the mail slot, saying something like:
“Mail for Ricki.”
“A letter for Ricki.”
“Special express telegram coming for Ricki.!!”
The child is then supposed to return the letter to you along with his answer. If the answer is wrong, say “Mistaken address!” and push it back through.
Ricki LOVES this game and has lots of fun while learning.