Hi Al,
I believe that each state actually dictates this law and in some states it depends upon the type of product you are selling. For example I know that in one state if you sell agricultural goods such as sod, trees etc via mail order or online, for those products you cannot charge the customer until the order is shipped because of the fact that if the customer orders during winter, the crop might not ship till next spring and because of weather that crop might not be able to ship at all.
I would check with your local commerce department or if you are signing up for a merchant account some where they should be able to provide that information for you for your state.
Joe