I have a Dahon Boardwalk, and when folded, it fits quite nicely under my bus seat. As for shopping carts, even though I can store it underneath, it often is too hard to get under there and out because of the pedals. Often I just but it in the basket (and get funny looks from the rest of the shoppers) If you are taking the bike on the bus, I'd suggest a bike WITHOUT an external shifting system. In my opinion one with a hub shifter would be better, as your bike WILL get banged up leaving you having to replace and repair the deraileur frequently (at least I have in my experience).

I love my Boardwalk. Its easy to ride and take care of. I've taken it up some pretty tough hills too, and although it takes effort, its still very doable.