Drupal and Ubercart both offer a lot of features out of the box. However, there's simply no way to accommodate every idea and possibility in the core of any project. As such, the functionality of these projects is meant to be expanded by contributed modules. You can find these primarily through the Downloads section of drupal.org.
This part of the User's Guide is meant to hold documentation for finding and using the various contributed modules created by the many Ubercart developers around the world.