Good job being cautious with all the scams going around. Amazon accounts getting "hacked", pretend credit card refunds, or someone posing as customer service helping you secure your "compromised" credit card. And of course the Nigerian prince needing 3,000 blue banners.
But yeah, as you've probably figured out by now, ACH is legit. I used to work at a company which regularly sent $50k+ payments with ACH. A couple of times a month.