Keeping The Customer Happy.

Happiness = Achievements/Expectations.
To keep your customers happy, you have to give them products or services that meet their needs. In this way you can seek to maximise the top half of the equation.

But the bottom half of the equation is also key and here , in order to maximise happiness, your goal should arguably be to minimise that expectations value.

You could do so by continually disappointing your customers, but that's not a strategy with legs. A better approach is to manage expectations, though not as an excuse for poor performance. Just make your marketing honest.