Regarding the eCommerce integrations, Jason is right that early on we wanted to partner with a few companies where we felt like we could deeply integrate and provide a controlled/polished experience for consumers. There is no exclusivity agreement with ShipStation (though they're a great choice for many) and we're actively working on APIs that will allow larger merchants to integrate with our order/inventory systems more deeply.
(Disclosure: I'm the founder/CEO of Squarespace)
But I agree with some of the comments. The restricted API kills all workflow-related processes. Regardless of shipping and fulfillment, there's tons of business processes that need to be done with orders and you can't do any of it with Squarespace. I've worked with dozens of clients who started on Squarespace and moved to other platforms simply because of Commerce restrictions.