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. Kind of like Dory in "Finding Nemo," AdBlock is always meeting old friends for the first time.
Similarly, uninstalling AdBlock removes all traces of it on your computer. "Reinstalling" is really "installing for the first time," because there's no way to leave a digital sticky note saying, "Hey, you've been here before."
Feel free just to close the payment page. 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 (choose the one that is appropriate for your browser):
I keep seeing your payment page or AdBlock is constantly uninstalled or disabled in Safari
I keep seeing your payment page or AdBlock keeps losing my settings in Microsoft Edge
I keep seeing your payment page or AdBlock keeps losing my settings in Firefox