You can test your code using the billwerk Sandbox without hitting the live system and potentially messing up your live account. The Sandbox is located at https://sandbox.billwerk.com, the corresponding API URLs start with
https://sandbox.billwerk.com/api/v1/. The sandbox and the live system are not connected in any way. Your live account is not known to the sandbox and vice versa.
Don't try to use the sandbox for live billing.
Sandbox accounts can be used as a staging system and they are free forever. The sandbox looks and behaves exactly as the live application, but receives updates a little earlier and doesn't have the same level of redundancy and availability. The sandbox doesn't support live billing.