Appointments+ selects the wrong service on confirmation/checkout

I've created a test page for Appointments+, however it's riddled with bugs.

http://demo.afspraken.org/

On the page, the default page created by Appointments+, you can select 'Gratis afspraak'. When you select an available slot, a different service is presented upon checkout/confirmation, the first one I've ever configured to be precise.

How is this possible?

Thanks!