There's no free lunch, let alone for a month.
To avoid this kind of dishonesty, then stop being greedy.
You don't have to be greedy to fall for false advertising. What if they told you that you could buy one and get one free, but when you actually buy it you are told that it's only times 4 so you have to buy 4 to get 4 free? Technically they aren't lying, but they aren't telling the whole truth. You could not need 4 of these items, or you could have not enough money on you to buy so many.
Be greedy, but read the small print.