Redirecting from sandbox PayPal confirmation

I have created the eCart pages using the eCart 4 Wizard.
From the checkout.php page when I press the Checkout with PayPal button I am redirected to the Sandbox PayPal confirmation page. After the confirmation I am redirected the pp_confirm.php page. After the submission of the order I am redirected to the pp_checkout_success.php page where I get the receipt. In this case everything goes well. The order is inserted into the order summary and details tables and the charges in the PayPal accounts are done. The strange in this procedure is that if I do sequential orders the value of the session orderID doesn’t change. The cart is emptied after the loading of the pp_checkout_success.php thus there session of the previous order is terminated but the user stays logged in.
From the checkout.php page when I complete the pay by credit card form and after the submission of the form I am redirected to the sandbox PayPal confirmation page. After the confirmation I am redirected to directly to the checkout_success.php page and not to the confirm.php page. The receipt that I get is correct regarding the sown information but without Order ID. The order is not inserted into the corresponding table and no charges are done in the PayPal site. I have defined as return url confirm.php. I have compared the configuration of the server behaviors of the pages that use the two different payment procedures, i.e. the check out by PayPal and the checkout by credit card and I cannot see any discrepancy.
Finally I am wondering if there is any reason to use the procedure “Pay by credit card” as this procedure provokes other problems of use (IPN handler etc)
Many thanks in advance