Can patrons have Postal Code rather than Zip Code?

You’ll have to modify the code. Look in includes/WEBLIB_Patrons_Admin.php,
function checkiteminform() and change this code:

$newzip = $_REQUEST['zip'];
    if (!($newzip != '' && (strlen($newzip) == 5 || strlen($newzip) == 10) &&
        preg_match('/\d+(-\d+)?/',$newzip) )) {
      $result .= '<br /><span id="error">'.__('Zip is invalid','web-librarian') . '</span>';
    }

You should change the match pattern passed to preg_match to match Canadian
postal code formats.

and in the function display_one_item_form() there is this code fragment:

<tr valign="top">
        <th scope="row"><label for="zip" style="width:20%;"><?php _e('Zip:','web-librarian'); ?></label></th>
        <td><input id="zip"
                   name="zip"
                   style="width:75%;"
                   maxlength="10"
                   value="<?php echo $this->viewitem->zip(); ?>"<?php echo $ro; ?> /></td></tr>

You want to change the label for the field.

If you have many changes, your best option would be to ‘fork’ the plugin and have a local copy on your desktop computer. There is a plugin that lets you update a plugin from an uploaded Zip file — this is very handy.
It would then be easy to download and patch new versions with your changes.