Great idea!
Can't tell from the photo, but it would be a good idea to keep the door on the garden mailbox in the closed position as critters would find it inviting. While I find it cute to lay my hand on a small green tree frog, I wouldn't want to disturb a wasp nest.
I keep a pair of garden gloves, my oldest pruning snips, an even older knife, some twine and a short claw type cultivator in my garden mail box.
