Commerce Checkout Login

This module extends Drupal Commerce with a checkout pane that allows users to log into their account, register a new account or (optionally) continue checkout without an account.

The original (1.x) version had a different approach:

Drupal Commerce comes with an Account Information pane on the checkout form where anonymous users must specify a contact e-mail address for their orders. This module extends that pane to provide e-mail validation and an inline login form via AJAX. If the e-mail address an anonymous user enters is already associated with a user account, the checkout form will expand with an optional password textfield and login button for immediate login.