Hi
I had Paypal express and paypal std enabled. I had the issue with the back button not working.
I then found issues with Paypal Express looping back to the log in page when trying to check out as guest so I removed it and just had paypal std and I set it as Auto Select. Again the back button doesn't do anything. This is not a big deal, but I figure a customer is going to click on it and say something eventually. I'd rather have a 'cancel' button that takes them back to the storefront. Or have the back button working...