certainly is at some airlines - I personally only took cash.... kidding - at my previous employer you could pay for an UG with a credit card at a rate which was about 1/10 the full fare price so for instance M class to J class was £600 and M Class to F Class was £800 and some variations in between.
It is/was a cheap but unconfirmed way to get a better seat however you couldn't always provide the upgrade if for instance catering/other products were not available onboard - not just a seat availability issue.