Hey there!
The app Coupon Carrier (https://apps.shopify.com/coupon-carrier) may be what you're looking for, which takes advantage of Shopify's Bulk Discounts app.
If you're looking to automate other workflows between MailChimp and your Shopify store, MailChimp has developed an app (https://apps.shopify.com/chimpified) that may help you as well!
Be sure to search 'MailChimp' in the Shopify App Store for more ideas. :)