"maidoari" is abbreviation of "maido arigatou gozaimasu"
you can hear this "maidoari" at vegetable store, fish store, long-established eating place(perhaps), and so on.I think it is usually used at market place.
on the other hand, in the case of "maido arigatou gozaimasu", you can hear at restaurant, department store, and so on.
the former is informal, and the latter is formal.