Talked to the guy this morning. He was very defensive and insisted that he's been selling nitrous since 1992 and he doesn't do business that way.
So in his humble opinion I must have opened the valve, or it leaks (!). Despite that the cylinder is still 21.70 lbs today, hasn't changed a gram. As if I wouldn't have noticed and heard 3 lbs of nitrous leaking into my car (bottles were in the back seat) during the 1 hr drive home.

All he would agree to is for me to come down and he'd fill it up in front of me.
Sure, another 80 mile round trip and $10 in gas. And the bottle's at his house, not his shop, more time wasted.

At BEST his scales are wrong - which probably means he's been shorting people for decades. I will be charitable and assume it's unintentional.
Credit card will only dispute all-or-nothing. I just want $22.75 back, not the whole $130. He did provide 80% of what I paid for.
At this point the overpriced $7.50 nitrous in my own town looks like a better deal
And now I know why, of 26 Google reviews, there are 21 five-star and 5 one-star. Live and learn.