shared ssl/ authorize.net question...

[razzo]

Hi,
I am running osc under a shared ssl cert.

When I enable secure checkout and the customer uses the Authorize.net module, it appears as if the session is not being transferred:

ie: OSC completes the transaction and returns to 'sucess.php', but the sale is not recorded under 'orders' in admin, nor is an email sent to customer or admin invoicing the sale.

Additionally, the customer shopping cart does not empty itself and the previously paid-for items are still there.

When the customer uses 'pay by check' option everything works beautifully, so I am suspecting it has to do with session storage not working when transfrred across from non-ssl to ssl to auth.net, and back to the cart.

Can someone tell me:
Is what I'm trying to do simply NOT POSSIBLE under a shared ssl cert?

Or is my configure.php set up set up correctly?
It looks like this:

// Define the webserver and path parameters
// * DIR_FS_* = Filesystem directories (local/physical)
// * DIR_WS_* = Webserver directories (virtual/URL)
define('HTTP_SERVER', 'http://mydomain.com');
define('HTTPS_SERVER', 'sharedsslURL.com/~myusername');
define('ENABLE_SSL', true); // secure webserver for checkout procedure?
define('HTTP_COOKIE_DOMAIN', 'http://mydomain.com');
define('HTTPS_COOKIE_DOMAIN', 'https://sharedsslURL.com/~myusername');
define('HTTP_COOKIE_PATH', '/shop/');
define('HTTPS_COOKIE_PATH', '/shop/');
define('DIR_WS_HTTP_CATALOG', '/shop/');
define('DIR_WS_HTTPS_CATALOG', '/shop/');
define('DIR_WS_IMAGES', 'https://sharedsslURL.com/~myusername/shop/images/');
define('DIR_WS_ICONS', 'https://sharedsslURL.com/~myusername/shop/images/icons/');
define('DIR_WS_INCLUDES', 'includes/');
define('DIR_WS_BOXES', DIR_WS_INCLUDES . 'boxes/');
define('DIR_WS_FUNCTIONS', DIR_WS_INCLUDES . 'functions/');
define('DIR_WS_CLASSES', DIR_WS_INCLUDES . 'classes/');
define('DIR_WS_MODULES', DIR_WS_INCLUDES . 'modules/');
define('DIR_WS_LANGUAGES', DIR_WS_INCLUDES . 'languages/');

Or Does this need to be changed?:

define('HTTP_COOKIE_PATH', '/shop/');
define('HTTPS_COOKIE_PATH', '/shop/');
define('DIR_WS_HTTP_CATALOG', '/shop/');
define('DIR_WS_HTTPS_CATALOG', '/shop/');
define('DIR_WS_IMAGES',

Or do my Admin/Session settings need to change?
They look like this:

Session Directory /tmp
Force Cookie Use False
Check SSL Session ID False
Check User Agent False
Check IP Address False
Prevent Spider Sessions True
Recreate Session False

Or, as I suspect, do I just have to pay for my own ssl cert to make this work?

Sorry for the long post....
Thanks in advance...