No, I don't have any add-ons that charge me. I tried reading how the free Dynos work but I didn't find anything regarding this issue. Here is an image
I'm no expert (just deployed my first app), but it seems that there's a clear indication of free or not in the overview tab for your app. For example, I see
on my app page. Presumably one would turn paid features on and off using the Configure Dynos link.