there are two ways to deal with Securing eCart:
1) Redirect all http traffic to the https server
2) Set the cart page to direct to https. to do this double click the Checkout server behavior on the cart display page and add the full URL To checkout.php.
depending on the gateway you select, you may have to find instances where you are redirecting the gateway to the http site again on the checkout pages and change them to point to the https server.
My personal preference is the first one. I feel safer shopping at a site that is on a secure server when I first arrive.