How old is the battery? In my experience if the alternator and starter test okay and all the connections are clean and tight but the battery is borderline, it needs to be replaced. It would be worthwhile to take the battery out and take it to them for charging and retesting. The only drawback is that most places do that indoors where the battery ends up warmer which increases it's output and isn't the best simulation of actual operating conditions in cool/cold weather.