Hi I used to take the shakes too, last year and I lost quite some weight. Actually when i weighed myself maybe I hadn't actually lost much but my clothes fitted better and I went down 2 sizes.
You are supposed to eat healthy but nothing wrong will happen if once a week you indulge yourself a little.
The second shake though I used to take together with some fruit (maybe an apple). I really felt a lot better after a few weeks, not only because of my weight but also my skin was healthier.
In any case the person who sold you the shakes should definitely have a lot more information than me...maybe they can guide you a little.