on the webassist/plugins/shopping_cart/process_transaction.php page, find the following line:
<?php
// if in test mode, skip all checkout code
if ($WAGLOBAL_Test_Mode != true) {
?>
and change it to:
<?php
// if in test mode, skip all checkout code
if ($WA_Store_Cart->GrandTotal() == 0) $WAGLOBAL_Test_Mode = true;
if ($WAGLOBAL_Test_Mode != true) {
?>