Clarification: That is to say, I put the product in my cart, enter my email address on the cart page, reenter the same email address, enter my billing information, click “Review you Order”, and that error message pops up at the top of the page. I’m sure that I entered my email correctly. I even made sure that there were no invisible spaces before or after.
No problem. When you request your key after the purchase with the mail address you want to have registered, simply include info about the actual purchase (purchase e-mail address or MyCommerce Order-ID).