Of all the scams out there that is a good one. Ask how much it would cost to flush the washer fluid? or the to send ionized air through the exaust system. Perhaps they could send a cleansing pulse of electricity through your wires or grease the window tracks. Maybe they should adjust the torque in the engin mounts.

Brake fluid doesn't get dirty and unless you have allowed fluid to get too low (by over bleeding) there won't be any air in the lines. So I would explain to them if they screwed up the brake job to the point they need to flush the lines then that should be complimentary service.