the checkout page is missing the following script tag in the head section:
the checkout page wont automatically remember the info entered if the modify button is clicked.
To make it remember, on the bindings tab, open the session collection,each form element has a corresponding session variable, the names begin with "eCar_checkout_form_" then the name of the element it corresponds to.
for each element, select it in design view, then in the property inspector, click the lightning bolt icon next to Initial Value, and select the corresponding session.
for the select lists, click the dynamic button, then next to Select Item Equal to, click the lightning bolt icon and select the corresponding session