I would like to confirm if this email is legitimate, or if it is a scam. They are following up quite frequently.
Email:
Hi @FortunateCO
It is the same if they wrote you from iamscammer@gmail.com , the user name does not matter, and Shopify will never use the GMail domain.
There are a lot of scams around, you can search discussions and find 1000 topics with the same or similar titles and content.
Stay safe and do not click any links in emails 
@FortunateCO Hey I can confirm that Shopify emails always come from a Shopify.com domain. The email you received is not from an official Shopify address, so it is likely a scam. Avoid responding and do not share any personal or account information.
I don’t think Shopify would use gmail for communication. See this reply if you want to find out more or report the email.