That's right, Squarespace does not support GTM implementation on their sites and therefore this is not supported on their checkout process, even if you're checking out users on your own domain, there's no control over what happens in the backend of that checkout page.
However, we've found a workaround for this issue which works perfectly for us.
So, the idea behind having Google Tag Manager on the site is so that you can track specific actions that you can't track by simply using Google Analytics - I take it that this is why you're also using GTM.
One of the first steps we take whenever setting up GTM is to setup a "page view" tag to send this data into Analytics, which I assume you've also done.
We also know that Squarespace and eCommerce tracking through GTM doesn't work, HOWEVER, eCommerce tracking through Google Analytics IS SUPPORTED by Squarespace (by adding the ID code within the web settings) so, the solution here is to use a combined approach by doing some of the tracking through GTM and some of it through the out of the box tracking with Squarespace and Google Analytics.
Follow these rough steps (I'm assuming you've got experience with GTM and that you've already done some of the implementation):
- Add your analytics tracking code to Squarespace settings
- Enable eCommerce tracking and enhanced eCommerce settings within your Google Analytics view
- Go to your GTM container and PAUSE the tag that's sending "Page View" data into Google Analytics
- Voila! 🙂
This way, Squarespace will take care of sending Page View and eCommerce data into your Google Analytics and you can do all of your behavioural tracking (button clicks, scrolling, Facebook Pixel, etc) within your GTM container.
I hope that's clear enough and that it works for you. Let me know if it does! 🙂