At first glance, this looks like a typical ANOVA question - you are examining if the mean of amount of purchase varies between four groups/conditions. Alternatively, you might use linear regression with a set of three dummy variables and a reference group/condition. Stata, SPSS or R take care of the estimation :-)
Inspect the histograms of amount of products bought. If they look quite normal indeed do run an ANOVA; if no, run the nonparametric Kruskal-Wallis test.