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

  1. On your catalog item widget, ctrl + right click the widget and open
    the widget instance options.
  2. Check "Show Add Cart Button" What you should

Adding "Cart" on the header

  1. From the Service Portal configuration page, select the Portal
  2. Goto the SP Header Menu
  3. in the Additional options section ensure you have the following
"enable_cart": {
"displayValue": "true",
"value": "true"