Digital proposed this back in 1997 or so (before Digital was swallowed up by Compaq, then by HP). They called it Millicent. http://web.archive.org/web/19970601153143/http://www.millice...

There's a patent and everything. http://www.patentstorm.us/patents/7110979.html

I think Stephen Glassman (formerly at DEC) is now at Google. Not sure if he is/was involved in Google Checkout.