You need to build an initializer like the documentation subscribes (look for the part where it describes a file in config/initializers, you can’t just cherry-pick from these docs, you have to follow them step-by-step in order to get the add-on to work). I figured out you weren’t properly hooked up because you didn’t reference the BONSAI_URL line in your code base anywhere.
As for whether you have to pay: looks like it. Folks don’t provide service for other people without hoping to get something in return! Maybe you can start with one model and then verify it works before signing up for a paid plan.