Hello,
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 st