I'm interested in this too. I tried using MailChimp's integration w/ code injection, but that only enabled pop up subscription and re-targeting. There is an option to inject code into the Squarespace order confirmation page, and the fact that it has a list of items you can pass on (customer email address, order id, order total, etc) makes me hopeful that this might work... but how to get it to work exceeds my coding knowledge.
I totally assumed that this would be possible with Zapier, but that's only the form submission block. I would appreciate any help here as well!