Why am I getting lots of $0 empty carts?

When a customer clicks on a link that you've sent through MailChimp, a cart is automatically created for the customer. This functionality is how MailChimp tracks conversion rates from the emails you sent out and is why sometimes your Store > Carts can sometimes register as lots of $0 carts.

In general, you should expect to see a lot of empty carts right after sending an email campaign to a list of customers.