There is no option to add-on extra data. Instead, you just use your rate plans overage rate on data. This can be found on self-serve under Plans & Add-ons. Your overage rate is plan specific. Some people have an overage rate of $10/GB and some have an overage rate of $10/250MB. It just depends on the plan. But regardless, there is no add-on and if you want to use more data, you just pay your overage rate.