So it seems the first thing a lot of places have to do when setting up
the service portal is to re-setup two-step-checkout.
Because I've had to do this a few times and don't like looking it up
here's the steps I've followed to set it up;
Adding "Add Cart" on the form
- On your catalog item widget, ctrl + right click the widget and open
the widget instance options.
- Check "Show Add Cart Button"
Adding "Cart" on the header
- From the Service Portal configuration page, select the Portal
- Goto the SP Header Menu
- in the Additional options section ensure you have the following