I have used Uprinting.com and like them very much. I have not tried their postcards yet though. They do a free proof and are generally less expensive. I looked at Moo postcards which for 60 cards is $49.99 ( I don't think this includes shipping), Uprinting is $42.01 (including shipping) for 100. Of course that is just the starting point, but you can definitely see the savings.
Good Luck!