How much bolt-on data can I purchase?  

You can buy bolt-ons in 1GB or 3GB of data. After you’ve purchased a bolt-on, you cannot buy another of the same amount until you’ve used up to 95% of the data from your existing bolt-on. If you’ve purchased 3 months on the $29 plan, you can buy 1GB of additional data after you’ve used 1GB of your total data.