Read more below to better understand the situations where the payment page appears.
When you install AdBlock
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 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.
If you've paid for Premium, you'll need to activate it any time you install AdBlock. Find steps to do this here: Unlock AdBlock Premium.
Otherwise, 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.
When we update the extension
If your extension has not been upgraded to Premium, when we update the extension (about every two weeks) we'll display the payment page to let you know about any new Premium features and offer you the option to upgrade. You are welcome to dismiss the page by clicking the "x" on the tab.
If you've paid for Premium and still see the payment page when we update, please make sure you've activated the upgrade in the extension: Unlock AdBlock Premium. This will stop the payment page from appearing during updates.
At this time, the only way to prevent the update page from appearing is to enable the Premium upgrade.
Alternate scenario: There's a problem with your browser
If our payment page opens every time you start your browser, we're not doing that intentionally. Something is causing AdBlock to return to a "just installed" state or clearing your settings. Please see this article: Seeing AdBlock's payment page more than once or AdBlock settings are changing.