The shop should tune it up better. You should be able to ride off curbs and the like without the chain jumping off the sprockets. The thing the chain is rubbing on is called the front derrailleur cage. You can trim the cage position with the the adjustment thimble at the shifter. But be sure that you're not trying to use a bad cross chain combination of front and rear gearing. The big ring at the front should only be used with the smaller 5 or 6 cogs at the rear. The middle ring at is OK with all but the smallest or largest of the rear gears and the small ring at the front should only be used with the larger 5 or 6 rear cogs. While this stuff isn't written in stone it's a good rule to ride by as it'll avoid the really bad cross chaining issues.