Get rid of username field. Customer should only enter an email address and password.
In e commerce it is very outdated for customers to have to create a username to buy a product online, it's just not practical in today's modern online environment.
Just get them to enter an email address and a password and that's it. Keep it simple for customers :)
Andrew Lansdowne commented
This is already available - settings - account - tick "When creating an account, automatically generate a username from the customer's email address"