In real world, a customer can have different contacts. We can allow these contacts to log-in to Web Store. If customers contact created a Sales Order, the transaction will be recorded in customer contact thus, the email will be sent to the Customers email and not the contact who created the Sales Order.
In Order to send the email to the contact of the customer who created the Sales Order we must use this below details:
New Action: Send Email
Trigger On: After Record Submit
Context: Web Store
From Field: Record: Current Record
With the above settings, the email created by your workflow will be sent to the Contact's email not the Customers email address.