I have a 1991 buick par avenue with 3800 engine.

If you have NOT replaced the CRANK sensor....REPLACE IT NOW.
It is known to fail often and cause stalling often.
It is so bad it should be replaced at every tune up.

Other common causes of stalling as you describe are.....failing fuel pump or connections to same.

A/C cycling lowering voltage and causing stalling?........not likely.
If you want to test that.....just disconnect the A/C pressure switch to prevent the A/C from coming on at all for a few days and see if that is causing a problem......before spending more $.

the price for the ac service sounds steep. The Stalling can be caused by ac but I have not came across this at high speed. Did they replace the Crank sensor?? Or the ICM? These can cause these issues. The crank sensor more so than the ICM. The cam sensor usually does not cause a stalling that I have ever dealt with. they will cause some issues but not at high speeds.