In my case though, I was actually using Optimizer with other credit card processors and they allowed me to tag those conversion pages on their server.

I think what you said above isn’t correct regarding Paypal. Even though you can have your test page and confirmation page on different domains, you still need control of all the pages leading to your conversion page. If you go through Paypal, Optimizer loses site of the fact that it is the same visitor, even if they come back to your site for the confirmation.

To verify this, I asked Google. Their answer: “It is necessary to add tags to all pages leading to the conversion page, so unfortunately it will not work for you because you will not be able to tag your PayPal pages.”

I’m planning on doing a little hack because I really want to be able to use Optimizer. I’m going to have my PAY button go to a page that redirects to Paypal. However the page is going to have the tags on it as if it is the confirmation page. So at least it will treat everyone who clicked on PAY as a conversion. Even if they end up bailing on the Paypal screens, at least I’ll have information on who attempted to pay, which should help me increase conversions.