Make use of drupal_prepare_form so hook_form_alter can be invoked

In uc_optional_checkout_review_form_submit(), the form is not called conform to the Drupal standards:

    $wps_form = uc_paypal_wps_form($form_state, $order);

Instead, use drupal_retrieve_form() and drupal_prepare_form so hook_form_alter can be triggered:

      $wps_form = drupal_retrieve_form('uc_paypal_wps_form', $form_state, $order);
      drupal_prepare_form('uc_paypal_wps_form', $wps_form, $form_state, $order);

Find patch attached.