The overhead running a legit business is unreal.
My general advice is don't use post mix concrete, use regular stone mix and backfill immediately. The backfilled dirt will hold it in place and slow the cure, giving you hours to go back and tweak as you go. That rapid post makes no sense, there's no urgency in the setting phase, that's the opposite of what you want.
And remember, if you do a good job it can last 20+ years, so don't be lazy and take a shortcut because it's "good enough". You're better spending $300 on a rental machine to dig the holes than to set a post that isn't deep enough. I'm in Canada where frost can be hell, my posts go 4' down and usually 5 bags concrete per post, then another 3 bags down the post once it's all assembled. Your 40' fence I could easily put in 50 bags. Don't base your shopping list on that, but know that if you care about longevity it takes patience and hard work, like anything else in life
Interesting story about a bit of a mean guy. Not too nice a fella, this rattlesnake