I don't think this can be done because the browser is seeing it as a unique URL and recording it to be helpful to the user. It's important for the browser to know the history, but I understand why you don't need to show this to the user.
If you weren't using the PayPal API, I'd suggest not using querytring values in the URL, but that's probably unavoidable in this case.
Sorry to say, but I think you're stuck with it. I never found a way around this in any PHP or Classic ASP applications.