I think they use credit cards for the larger transactions in FF7 and FF8 ;]. They had used Gold Pieces prior to FF7. I think the reason the changed is that FF7's world is a lot more advanced, so using gold pieces would seem kinda out of place. So yeah, in the more advanced worlds, I think they use notes and credit cards in addition to regular coins. Maybe the Throw materia exchanges notes into coins as a part of its power :p.