The sketch has been done several times since then. Feldman did it on a UK TV special with John Junkin, then on US TV with Flip Wilson. This version didn't work at all -- Flip is far too hip to be acceptable as an uptight sales clerk. Another failed version had Cleese teamed with his ex-wife Connie Booth. She just isn't eccentric enough.
There have been other Python remakes, including Cleese and Chapman for an LP and Cleese with Eric Idle at a recent reading to promote the former's autobiography. Despite the sketch's link with Feldman, I think Idle is my favorite customer -- his amiably cooperative but ultimately absurd actions nicely contrast with the ever-increasing craziness of the clerk.
The version I'm posting is one of the more obscure, from a short-lived NBC variety series called The Big Show (1980). Graham Chapman plays the clerk, with English impressionist Joey Baker (some will recall him from the cult series The Kopykats) as the customer. He's not bad (even if he's clearly reading off cue cards), but this is Chapman's sketch, as he throws subtlety to the winds and goes far over the top, giving every line everything he's got. It's probably the most "Cleese"-like performance Chapman ever gave.