Blinker fluid is a critical component of your vehicle. Don't try to change it yourself, but that shop sounds fishy to me - you can't just "top it off." If your blinker fluid is low, you really need to empty and flush the whole system, check for leaks, test the blinker fluid pump, and assuming no system issues are found, finally refill with fresh blinker fluid specific to your make and model of vehicle. Take it to a more reputable shop and insist you need all of this done - if they resist, tell them you know it is a lot of time and work but it isn't as if you won't pay them for the labor. They may need to keep your vehicle a couple days, so be prepared for that as well. Good luck and let us know how it goes!
