For the U.S. Postal Service, the words “digital” and “opportunity” are two words that don’t normally go together. After all, the mail carrier has lost billions of dollars in revenue to customers’ growing fondness for Internet bill-paying, electronic greeting cards and so forth. Last month, however, Postmaster General Pat Donahoe announced the launch of a “digital solutions group” intended to sniff out potential money-makers in the online sphere. More recently, the USPS inspector general has singled out one in particular: Putting the agency in the digital authentication business. You can read the IG’s full report here, but in a nutshell,…