"I've read in a few places that after the war, the pickers were paid by the sack they turned in or by the pound."

I well remember back in the 50s, when I was a small child at my grandparents, watching the pickers come in at the end of the day and their sacks being weighed so that they could be paid properly. And how my grandfather would watch closely when they'd dump the sacks to make sure no one had tossed in any brick chunks or old bolts or such to increase the weight. The cotton would sit in a wagon under the barn until it was full, then they'd hitch up the mules and haul it to the gin. A full wagon would be a good bale of cotton. We used to like to play in the wagon and tunnel into the cotton. And woe to the child who knocked so much as a lock of cotton from the wagon onto the ground.
By the 1960s, mechanical pickers had pretty much put handpickers out of business, although I think the prisoners at Angola picked by hand for a while after that.