The postal workers, who ship mail for a living, really should have advised him better.
That's like saying the people taking orders at McDonald's make food for a living. While there's of course exceptions I generally assume retail clerks don't have any real experience with any other part of the business than pointing out where things are, pushing the products and accessories the company wants to sell and working the cash register. The real skilled people are often working somewhere else, the front line staff is often temps and extras or quite happy with jobs where they don't have to think so hard. Not that I really blame them, but I'd rather set my expectations low and be positively surprised instead of the other way around.