It's really dependent upon what is considered problematic drinkers.  Digging into this study, they're saying 69% of alcohol buyers are "hazardous or increasing risk" when it comes to their health and drinking.  But the measure of that is 14 or more "units" per week for women and 21 for men, which in US talk translates to about 6 beers per week and 9 beers per week respectively.  So, a woman who comes to your store and buys a 30 pack once a month is considered "hazardous or increased risk".