Hobby Lobby also had a denim satchel type bag on clearance for $3 and a little foam hat for $0.99. This picture doesn't show it but we used some white stick on felt to write MAIL on the bag and MAILMAN on the hat. When Jacey puts on his mailman uniform, he "requires" that everyone call him Sir. Don't you dare call him Sir when he's not wearing it though!
I also found a sorting/filing box at Office Depot for $13. We put each family member's name on the slots so that he could write letters and deliver it to their individual boxes. I put together a box of envelopes, postcards, paper, markers, sticky backed foam shaped like stamps, and To: and From: stickers and he went to town. I explained to him how people write the letters, put on the address labels and stamps, then drop it in the box. Then the mailman (or woman) comes along and sorts it and later delivers it. He practiced this all week and was quite the stickler when daddy didn't do it in the proper order.