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)