If you are getting an error when trying to upgrade or get a "No offers found" message, this usually solved by allowing the Phone Number permission through the Phone settings for the app for carrier billing to be allowed.
1.) Select your phone's Settings app.
2.) In the Settings menu, select the Apps/Application Manager section.
3.) Find Slacker Radio and tap to open.
4.) Allow the Phone permission so that the carrier can bill you on the mobile bill.
If it does not help, contact Slacker Support directly.