Do I need a purchase to use the beta?

No. Beta-only features are free to use and you don't need any purchases to use them; regular (already-released) features are still paid, even in the beta. If you already have an active subscription from the regular version, it will also work in the beta.

Note for iOS users: due to the way Apple does things, we're not able to offer purchases through the beta. In order to make a real purchase (which would unlock all functionality in the regular version and the beta), you must do it through the App Store or through the regular (non-beta) version of Carista. We're working on ways to improve this process.