How to Double-Opt in "Accepts Marketing" on Shopify checkout even if not purchased?

Hi, 

I want to start an abandoned checkout flow using Klaviyo. I want to send out my abandoned cart email to anyone who accepted marketing on checkout and double opt-inned. 

What I would need is, that once a customer types in his email and proceeds to the next step in the checkout process klavyo automatically sends out the “confirmation mail” (obviously only when having the “accept Marketing box” checked).

I can only find solutions to send the abandoned cart email out to people who signed up through another list. 

how do I integrate the double opt in email for the checkout emails? I cannot find a solution for this. 

thank you