Short answer
HubSpot's KB: needs Shopify via data sync or the Carts API. Cart emails work only in cart-based workflows, max 250,000 recipients per hour, and can't be A/B tested.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Test with a real abandoned cart before turning on.
3. Related
The cart object and Shopify sync.