Temperature gauge going up and down when driving.mechanic said maybe not enough coolant.wanted 90 to diagnose the problem.

 

Hello an Welcome to the MCF! So you saved $90, sometimes it is more efficient to have a shop do the troubleshooting. This seems relatively simple. Even if you coolant was low, it would blow hot air, probably way bubbling hot air...

I will go out on a limb and say that the problem is your thermostat has failed in the open position. They are easy to remove and visually inspect., when cold and the valve has frozen in the open position you can see a space between the valve and the T-stat body assembly. Watch a couple you tube video about the T-Stat you will be a pro in no time...

 

If your shop does it right $90 is not a bad price to diagnose. That is if it is done right. If he just looks at it and makes a guess, it is a waste.
Here are some things I have found over the years for loss of heat and temp to fluctuate.
Low water will cause this. The temp gauge will only read water temp, it cant read air temp. If it reads too cool it could be air in the system from low coolant. The loss of heat in the climate control also could be low coolant. Air circulating through the system due to low coolant. The heat works by the heater core filling with hot engine coolant and a fan pulling air through the core into the car. Air in the heater core will allow it to cool down and blow cooler air.
A bad thermostadt is the most likely problem. When they stick closed the engine will overheat because the coolant will hang in the engine for too long. If it is stuck open it will run cold because the coolant circulates too slow. The coolant will spend too much time in the radiator and cool too much.
I have seen many engines loose heat in the climate control and the temp fluctuate with blown head gaskets. I have even seen a couple of engines appear to be running cold on the gauge, heater blowing semi warm air and blowing white fog out of the exhaust. These have ended up being cracked heads, cracked block or blown head gaskets.
Most likely your looking at a bad thermostadt. But to test this correctly the mechanic should do a pressure test and leakdown test on the radiator. He should test the thermostadt. He should test the climate control system itself to make sure it is functioning properly, but I am guessing it is if your gauge is fluctuating.
If the mechanic finds the cooling system is not holding pressure then he will need to look for where the air is escaping, and if it were me I would also followup with a compression test.
That is to check it properly. Most likely he will just check coolant level then swap the thermostat and see what happens.

 

The info that Zippy and SteveL have given you is sound advice, the component that came up was (heater core). Just for sake of finding out if this could be the culprit check the floor under the glove box area and back to the console and see if the carpet is wet. If it is it could be the heater core just in the beginning stages of going bad. I have had a few of those go bad and you won't get any heat then either.
Like ZIPPY indicated it is good to check youtube there might be something there that is a perfect fit.
good luck!