looks possibly fake...
but its possible, too. The stem mounted to the handle of the shopping cart and the fork mounted to a lower crossmember would keep the front end of the cart down, if done right. Good shopping carts can be ridden empty...I used to work at a grocery store in highschool (when I was 180lbs) and it was no problem. Handling would be iffy, but fitting bigger wheels would help. Shopping carts are strong as hell by design...Migrant workers proved that to me (my GOD the amount of food they fit in them!)
I say, Plausible....yea I love mythbusters...maybe they should do this one!!!!