Heat and A/C coms out of every vent!

What is your question?

I realized there was something wrong with my A/C this summer because the flow was so weak; the reason was because my car was pumping air out of EVERY vent; upper, lower, and windshield. I tried replacing the Heating & A/C control that is in the dash, but that did nothing. The heat and A/C do work, but the control to direct where the air should go isn't working, so like I said it just goes out everywhere. Someone told me it sounds like the "doors" on the "air handler box", but I don't know what that means! Is this the problem, or could it be something else? One thing I know it isn't is the blower motor, because someone already checked that. Can someone explain to me: 1) What I should tell the mechanic 2) How much it SHOULD cost to repair since mechanics are always ripping me off. 3) How I can make sure the mechanic doesn't up sell me on more replacement (if any) parts than I need? Any thoughts or ideas would be greatly appreciated! Thank you!!!!