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.
A minimalistic signup form built with HTML5, jQuery and ... nothing else. Less than 100 lines.
A more complex signup form built with AngularJS.
Simple signup built with HTML5, jQuery.
Simple Backend and Admin UI to illustrate REST API usage and webhook handling.