use the ecart_checkout form (checkout.php) binding group.
If you are using Spry validation, client side validation is not needed. They both use javascript to perform validation in the clients browser
for the validation trigger, click the lightning bolt icon and select the submit button from the ecart_checkout form (checkout.php) binding group.
failed redirect should point back to the checkout.php page.
see this thread for more details on using server validation with the eCart cehckout pages:
showthread.php?t=21834