How do I remove a page from the browser history?

window.location.replace(URL);

window.location:

replace(url)

Replace the current document with the one at the provided URL. The difference from the assign() method is that after using replace() the current page will not be saved in session history, meaning the user won't be able to use the Back button to navigate to it.

As a web application, you'll never have full control of the user's browser. Even if there was a way to instruct the browser to not store the page in history, which I doubt, you can't be sure it'll work. For example, a clever user could tweak an open-source browser to store every page in history, no matter what.

I think you should try to approach the problem from another angle. You could, for example, detect that it's the same form which is being forwarded and not send it to paypal the second time. The important thing is to do it server-side.

I'm not sure if that can be done. But here is an idea how you could prevent that resubmit of the form.

You could insert a hidden input in your form which at the beginning would be empty. On submit you'll write a value in that field and make sure you check on every submit attempt if this field is empty.

If it is not empty on submit you know that the form was previously sent and you could warn the user.