Stephen,
I am having similar issues. My shop is shop.fosterscreations.us I was using sfoster @fosterscreations.us for my Shopify correspondence. The problem is I can only have one set of DMARC Records . I utilize that email for mailchimp and have created the DMRC records to their specifications. Shopify is still saying no records found so I changed my shop correspondence and send to customer email to shop @fosterscrestions.com (as that domain permanently redirects to fosterscreations.us and I can have multiple email addresses there. My fosterscreations.us email is with Office 365 via godaddy and the plan only allows 1 email for the account) is there a way to use the same DMARC records for Shopify and Mailchimp? I can’t ask Mailchimp as my plan does not come with email support. I am about to give up on the hole thing and have my customer emails come from Shopify.