Plugin needs to directly load a file

I’m writing a plugin that sells subscriptions for a product using paypal.
The PayPal IPN needs to return and process the information to a script. now, in that page, i need to ability to update options in the WordPress database, I was thinking of just including the wp-config.php file, and writing custom database queries, but the problem is that i can’t even include that because i have no way of knowing the url for the installation (this plugin needs to wok on different installations, so nothing can be hard-coded.

Any suggestions?