How can I indicate if our donors prefer to receive Charitable/Political Tax Receipts by Email or Postal Mail?
The CDN Tax Receipts extension contains several hooks - one of them allows you to set the 'method' of delivery (Email vs Print) for a specific Contact. Here's an example implementation:

/**
 * Determine Contact's Preferred Communication Method
 * return 'print' -&gt; if Postal Mail is checked
 */
function my_receipts_cdntaxreceipts_method($contactId) {

  // Check if Contact's Preferred Communication Method = Postal Mail -&gt; id = 3
  $result_pcm_id = civicrm_api3('Contact', 'get', array(
    'return' =&gt; "preferred_communication_method",
    'id' =&gt; $contactId,
  ));

  if ($result_pcm_id['is_error'] != 1) {
    if (count($result_pcm_id['values'][$contactId]['preferred_communication_method']) == 1) {
      if ($result_pcm_id['values'][$contactId]['preferred_communication_method'][0] == 3) {
        return array('print');
      }
    }
  }
}