The CyberSource module includes integration of the Silent Order POST method, which requires you to do a couple things in your CyberSource account. When you signed up with CyberSource you should have received your password and URL for accessing your CyberSource account on the CyberSource website.
Log in to the CyberSource Business Center https://businesscenter.cybersource.com/ using your CyberSource merchant ID. Navigate to Settings > Security Keys. (If you do not see these settings, call CyberSource and ask them to enable the HOP setings). On this page, choose 'PHP' and click the Generate Script button. Place the HOP.php file in the ubercart/payment/uc_cybersource directory. The HOP.php file contains a function named L() that conflicts with a Drupal core function of the same name, so you'll have to make a small change in HOP.php to get things to run properly. Just open the HOP.php file and do a find/replace to change all instances of L( to csL(.
You must also adjust the Hosted Order Page settings. Specifically you need set the Receipt Response URL and Decline Response URL to http://www.example.com/cs/receipt and check the box beneath both those textfields. This really just reports to a dummy URL defined by the module, but you have to check the checkboxes so the module can receive the transaction results and act on them. Use the same URL as your Merchant POST URL.
Once you've done these things, you should be ready to test it out! When testing with CyberSource's Silent Order POST service, you can't process anything over $1,000. This limitation does not apply in production mode.