you can get the Express Checkout transaction ID binding by clicking the plus button on the bindings panel and selecting eCart -> PayPal Express Checkout -> Process transaction With Paypal Express Checkout.
And that didn't work either. I plugged all that in and ran a purchase through. When it came back to my site, it redirected to the pp_checkout_failure page. Not only that but it charged my credit card. Which turns a nuisance into a serious problem. :-(
I pulled it all back out and I'll just leave the page as it is for now. When I get an order in, I'll just go in and update the database by hand. It's simpler that way than trying to wrestle this bear.
Also, Jason, I really appreciate your help but all you've been doing is guessing on this one. Please don't do that to the next guy. It undermines the faith we have in you to help us. Hell, I can guess wrong just as badly as you can. We depend on you for answers, not guesses.