First, thanks for your support. We really appreciate it!
Our payment page opens every time AdBlock is installed. That's because the only way we keep track of users of our free products is with an anonymous userID, which is randomly generated anew every time AdBlock is installed. Each installation erases all previous userIDs. Basically, we're tracking installations, not users. We have no way of associating a new installation with a payment for a previous installation, or an individual user. Similarly, uninstalling AdBlock removes all traces of it on your computer.
Feel free just to close the payment page by clicking the "x" on the tab. You don't even have to wait for it to load. AdBlock doesn't need the page open to finish its post-installation setup.
If our payment page opens every time you start your browser, please see this article: Seeing AdBlock's payment page more than once or AdBlock settings are changing.