You can use field validation to capture specific information. For example, to add an additional required field to the billing addresses to capture an e-mail address, add code similar to the following on your billing page:

<span class="atg_messaging_requiredIndicator"
  id="emailValidatorAlert">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span>
 <dsp:input type="text" name="emailAddress"
  bean="BillingFormHandler.emailAddress" required="<%=true%>"
  size="25" maxlength="25">
    <dsp:tagAttribute name="dojoType" value="EmailTextbox" />
    <dsp:tagAttribute name="required" value="true" />
    <dsp:tagAttribute name="missingMessage" value="${emailMissing
      }" />
    <dsp:tagAttribute name="invalidMessage"
      value="${emailInvalid}"/>
    <dsp:tagAttribute name="inlineIndicator"
      value="emailValidatorAlert" />
  </dsp:input>

Additionally, you must have a form handler equivalent to BillingFormHandler in the example that accepts the e-mail address as one of its inputs.