in the post
1. In the process of being sent or delivered by the public mail system. Primarily heard in UK. I hope my mother's letter hasn't gotten lost in the post! They said that my work permit is in the post. I hope it arrives soon!
2. In a letterbox or other point of collection for delivery by the public mail system. Primarily heard in UK. I put your cheque in the post yesterday, so it should reach you the end of the week.