What happens if I go over my storage quota by purchasing music?

The music store will permit you to buy more music when you are over your storage quota. If you are over your storage quota, you will not be able to add more files to your account from your desktop or from the web until you remove enough files from sync that you are under your quota.

You can choose to remove purchased songs from Ubuntu One sync. This can be done by copying them from your Rhythmbox library to a non-synchronizing folder (e.g., ~/Desktop). Once a copy exists outside of Rhythmbox, you can move the synchronizing purchased music files from your Rhythmbox library to the trash. Then you can move the copied files to your typical music storage location (e.g., ~/Music) and add them back to the Rhythmbox library. I will add an FAQ item explaining this process.