I think in order to know if the two drink minimum is sufficient, you need to look at your daily cost to operate your store, and determine how much of your business are those people who sit and read without buying anything. When you look at it, you might find out that its not worth it to press those few people into paying to stay because other customers are covering their expenses-- or you might find out that you do need to develop a plan to convert those "sitters" into purchasers.

I have an article on my blog that shows an easy way to calculate and track daily operating expenses and how to set sales goals for your shop. You might want to check it out-- it could help you determine how much those "sitters" cost! Here's the link... http://toolkit.imageworkbook.com