I post a fair amount of letters, and the kids love helping me post them. Woe betide me only having one item to post though, too much stress!
So we made our own postbox from a shoe box. The box itself was too shiny, and had writing all over it. So I covered it with plain white stickers, and then coloured the stickers in red. I cut a posting slot in the front, and made sure the lid could come off easily.
Next, we needed something to post, so we took some postcards, and Hannah helped me address them. Great writing practise! Hours of fun posting the postcards, and they both get a turn!