There are easier and safer ways of robbing shops. By the time you're level 18, you should have access to most of them, I would think, or, more likely, you have good enough equipment that there's nothing worth robbing except the Casino. I don't know how reliable this method is--I usually consider it a nuisance because it means you have to go through 500 extra prompts if you want to check out all of the items in the Casino.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.