title: Websphere Commerce shipping charge customization
body: <p>I'm trying to customize a shipping charge processing in a way that it is applied only if a textbox field was filled. </p>
  
  <p>I´m having a hard time figuring out the right place to put the snippet that solves this problem.</p>
  
  <p>As I understand, I need to extend a class that have the shipping charge value available and it's value has not been applied yet, so I can change it to zero (invalidating that charge) when the field was not filled. I already read a lot of the <a href="http://publib.boulder.ibm.com/infocenter/wchelp/v7r0m0/topic/com.ibm.commerce.calculationframework.doc/concepts/ccfintro.htm" rel="nofollow">Calculation Framework</a> and debbuged a lot of the default implementations, but no success yet.</p>
  
  <p>Any tips? Thanks in advance.</p>