What if the defect is in the original product, such as Drupal, osCommerce, etc.?

With software that is not written by us, we will seek a solution to the defect, but are not responsible for it, any more than we would be responsible for fixing a defect with Microsoft Windows. A benefit of Open Source is that a large, vibrant community contributes to it, and that it is free. However, there is also the fact that because it is written by volunteers, there is no one in particular to complain to when something doesn't work...bug fixes are done by volunteers as well, in their own time. Of course, the community's concern about code defects will typically be stronger, and resolution much faster, than were you to report a bug Windows bug!