One Ubercart 2 Gotcha - "Unable to send e-mail. Please contact the site administrator if the problem persists." - Drupal 6

When using ubercart as your shopping cart, after successfully checkout and paid, you get the following error message:
Unable to send e-mail. Please contact the site administrator if the problem persists.

The first thing come to your mind maybe, oh, my system can not send email?!, but I know that was not the case, since all registeration, friend invitation, emails were sent out with out a glitch. So, it is ubercart related.

It is easy to overlook some small things when one configuring ubercart, one thing is to fail to provide an email address for the store. So got to admin/store/settings/store -- set up the email account on this and correct it all.