@Crank are you mech savvy?
Did the bike shop pull it apart and look at it?
Freehubs are pretty simple to pull apart, grease up and put back together if you have the right tools.
Are you sure its a pawl based hub and not a star rachet? I know Bontrager uses DT Swiss internals or at least a similar rachet system and these have a tendency to free spin in both directions if the spring wears out or if the freehub isn't pressed all the way down on the hub.
You can get a decent wheel on ebay or new for $300. Just make sure it matches what you have now, through axle or quick release. I believe the rear axle should be 135mm wide. If you can't or are unwilling to swap out your rear wheel, move the disc over and remount the rear wheel and center up your Calipers just pay the shop and get it done. It's about an hours work. 70 seems high but if they warranty it and you can bring it back for any reason it's worth paying if you don't have the time/tools to fix it yourself.