well, really, you dont need to even log in, you just need to click the link in the email, then click the add to cart button on the page that comes up.
the WA_eCart folder will be created in the sites that you create with eCart. it will be created at the root of the site.
thje WA_eCart/WA_eCart_USPS_PHP.php file will only be created after you have run the checkout wizard and have selected to use USPS Shipping rate lookup.