Apps cannot delete items, including those stored by the Postmaster. You can retrieve items from the Postmaster to your character inventory with them, though. This is so trolls cannot go deleting someone else's items if a troll gains access to their characters through any of the Apps. If you're away from your console, then you don't have to worry about anything going to your Postmaster, since you're not playing, so there's no real need for you to delete anything form the Postmaster. If you're expecting to receive something in the Postmaster, you don't have to worry, as it will delete the oldest items (in order) to receive new ones. No manual deletion necessary on your part is needed.