eCart 6 uses a new persistence architecture. so to return a value from the checkout form, use:
<?php echo WA_getSavedFormValue('FormName','ElementName'); ?>
so for example if the form is named "CheckoutWizard_NewFromBlank_Default" and you want the email address:
<?php echo WA_getSavedFormValue('CheckoutWizard_NewFromBlank_Default','Email_Address'); ?>