Sometimes you buy something and you get less than you expected; you then have every reason to be disappointed. Sometimes you buy something and get nothing, which might make you angry.
Sometimes you buy something and get more than you expected. You might then be happy and/or suspicious.
Enter the new, brilliant marketing system: You buy one, and you get one. No more, no less, just the precise amount you paid for.
Think about it.
You might be tempted to dismiss this as trivial, but don't be so hasty! It takes time to fully grasp the implications. This is in fact a superior achievement, somewhere between the simplest possible sophistication and the most sophisticated form of simplicity.