Have you built a compelling Marketing Cloud email that conveys the exact message you want it to? Be sure to test it on desktop and mobile before your final send.
If your eComm user suddenly cannot send emails from Marketing Cloud, you may need to verify that their Marketing Cloud user is still integrated with Salesforce so that they can access the reports they need to complete the send.
One of the cornerstones of eComm is allowing constituents to manage their own email preferences. Learn how Salesforce and Marketing Cloud make that constituent-centered focus possible.