I've grubbed around (on drupal.org and on my drupal setup) for a while without finding anything. When I return to merchant from paypal I get a duplication in my url i.e. instead of
http://www.barkmail.com/drupal470/
I get
http://www.barkmail.com/drupal470/drupal470/
It's on IIS shared hosting. I don't seem to have the same trouble with various apache set ups or with 4.6 on the same machine.

Any suggestions?