In the dashboard view of your account you will see an “Add More” option displayed under your talk, text, and data usage. Choose the add on amount of minutes, messages, or data and complete the checkout process.
Please note, all add ons have their own 30 day expiration. This means that add ons will roll-over with your next billing cycle until they hit their 30 expiration date.
There are no extra fees for “topping up” your account nor are there any limits on how many times you can top up.