Drip's Shopify integration uses events to know what's going on in Shopify. When a customer makes a purchase, Drip sees that event and can then trigger your marketing automation rules.
It's important to know which events are supported so you can plan your store's integration. I've included the full list below showing the Shopify event that happens and the corresponding event name in Drip. You need to use the Drip event name with the
Performed a custom event as the trigger in your automation rules.
This list of Shopify events that Drip supports will give you the ability to add marketing automation around most of the customer actions in Shopify.