I don't understand. If you have their username, I'm saying you can look up their email address in the payments page. So then you would just send them an email.
Viewing post in Is there any way to contact customers?
You have to hover over the payment and click details, then the username is shown beside the email. The date on the review should help you find the right payment. I agree that responses to reviews would be nice, as would direct messages.
Congrats on selling so many by the way.