It is a CRM, Any form interaction adds a person to the system and makes an account.
But it does not mean they have to log in etc.
You can build your payment form so that it is all smooth and for a one timer. It just makes an account. The orders need this to attach to otherwise the system would not work for the admin of the site to process.
If someone goes to the site again they can either buy and enter their details again or if they want to use the login they can do. You can set the form to pre-populate then. If they use the same email the system will neatly attach the new order to the existing person.
Getting the form right front end is the only bit to worry about, I think in this aspect the system works very well.