Same goes for a shopping trolley. It is like a moving set of handlebars with a really stable base. The advantage of using a shopping trolley over a bike, is that there is plenty of clearance for the pedals, because with a bike you might clash your pedals with the bike pedals while learning. There is no substitute for learning in the open though I think, riding against a wall or with any type of crutch is only useful for getting the hang of rolling with the wheel.
I don’t know about using shopping carts for balance help but, they are really fun to use in a 2 man 2 team race around the shopping center. One person sits in the cart with their legs hanging out and the uni in riding position also hanging out of the cart. The other teammate gets behind the cart and pushes (on his uni of course). you do one lap and then switch places, and then do another. Whoever finishes first wins! As for rules, the only rule is that you have to stay on your uni. You can bash into your competitors all you want. This game is best played late at night when there are no cars or customers.