From AT&T’s website:
The 500GB Add-on provides up to 500GB of storage when using the AT&T Personal Cloud app. The app provides a way to safely store and protect valuable mobile content in the cloud. Through the app, customers can backup, sync, restore, access, create, and share their content.
Add-On will automatically renew each month at the then prevailing rate (currently $4.99/mo.) unless you cancel, and you authorize AT&T to deduct that amount from your prepaid account balance each month. You can elect not to Auto Renew by going to “Add-Ons” and managing your Auto Renew setting anytime through att.com/myprepaid . For more info go to att.com/prepaidcloud .