AdBlock is constantly uninstalled or disabled
Has AdBlock vanished every time you open Safari? Open Safari > Preferences > Extensions and see whether AdBlock is still there and has simply been disabled or has been uninstalled entirely. Either way, it could mean you have malware. Macs are vulnerable to malware just as Windows computers are, it's just more rare. And malware extensions often look for and uninstall other extensions that can keep them from doing their nefarious work on your computer.
If AdBlock is constantly being uninstalled when you open Safari, the first thing you should do is scan your Mac for malware. Follow the instructions in this Knowledge Base article: How to remove malware or adware from your computer
AdBlock's settings are being reset
Do you keep seeing our payment page or are AdBlock's settings being changed back to the defaults every time you open Safari? It could mean that either your installation of AdBlock has broken or your Mac's Keychain entry for Safari has.
First, try a clean, "from scratch" installation of AdBlock. If that doesn't help, try fixing your Keychain. Warning: This will uninstall all of your Safari extensions. Make a note of what they are so you can reinstall them later. You'll find instructions for both procedures in this Knowledge Base article: Safari can't install or update AdBlock
By the way, our payment page opens every time AdBlock is first installed. This is why constantly seeing that page is an indication that AdBlock's settings are being reset. One of those settings tells AdBlock it has just been installed for the first time. That is the only time it should appear. If you prefer not to pay for AdBlock just close the payment page.
Look for a problem with Safari
If the above suggestions haven't fixed the problem, then the problem is most likely Safari itself. Please follow the instructions in this Knowledge Base article: Basic Safari troubleshooting