you would need to add a select list on the checkout page with the service type options:
<select name="AusPost_ServiceType" id="AusPostService">
<option value="STANDARD">Standard</option>
<option value="EXPRESS">Express</option>
</select>
and add a hidden form element on the confirm page to capture the selection:
<input name="AusPost_ServiceType" type="hidden" id="AusPost_ServiceType" value="<?php echo((isset($_POST["AusPost_ServiceType"]))?$_POST["AusPost_ServiceType"]:"") ?>">
then on the confirm page, go to the server behaviors panel and click the plus button. Select eCart -> General -> Set Session Value.
set the trigger to any form post.
set the name to AusPost_ServiceType
for the value, click the lightning bolt and select the AusPost_ServiceType element from the form collection.
Now edit the Autralia post lookup server behavior, click the lightning bolt next to service type and select the AusPost_ServiceType session variable.