There's two big problems with that: 1.) Consumers like the amount they pay to be predictable - they will pay more to avoid surprises. Your scheme sets them up for surprise bills. 2.) There is a material cost for every customer who is surprised and then calls your CSRs to complain. That cost can be more than your entire per-customer profit for one (or several) months. (This is also why cell phone companies love to move to flat rate billing where they don't have to argue about bills as much.)