Thanks, will look. Last I checked you could put a tracking pixel on the confirmation page that appeared after a purchase was made, but not in the intermediate checkout steps.
If this is possible it's another example of poor product marketing: I've asked for this feature multiple times via email and forums over the last 6 years, and I hear about it from HN? That's a broken experience for a customer who's paid them approaching 5 figures.
They try to make it slightly harder to write a store that steals credit cards.. not a terrible thing..