It looks like those apps may be more targeted towards marketing, and not for followup emails. I wouldn’t think it would be too difficult to implement with Flow, considering Shopify already has the ability to send followup emails to customers. Even if there was an automated way to send followups based on simple criteria, like shipping country.
Are there currently any plans to add features like that to Flow? Seems a little silly to pay a 3rd party developer for such a basic feature.