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
see

Adding “Cart” on the header

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

Docs