Yep! My customers so far have been really happy. :)
To compare to other services: the Chrome Web Store fee was 5%, Gumroad's fee is 5% + the Stripe fee, and my main competitor is charging 6.5% + stripe fee. Pretty standard. Why do you feel it's extremely high?
And if you think about the value of ExtensionPay... it saves people at least $5 / month on server costs, plus the time/ energy/hassle of creating their own payment system (which involves creating a user management, integrating it securely with the extension itself, figuring out how to integrate with a payment processor, setting up a database, sending emails, managing a server etc etc). Plus a lot of my customers are indie developers with limited resources that just want to focus on their extension.