SMS turned me on to the "Pool Store" in Saratoga, which runs a "special"
all the time on HASA 12% liquid chlorine (I buy 16 gallons at a time),
which has the following math, if interested.

HISTORY:
It used to be "buy one case, get one case free", which was pretty simple
because one case of 4 gallons was $18.40 + 8.75% sales tax added $1.61
which made each gallon $4.60 + ~$ 0.41 tax ~= $5.00/gallon out the door
if you only bought one case, but, with the free case, that added just
the ~$1.61 tax (California makes you pay tax on free stuff), so for 8
gallons, it used to be simple at $18.40 + 2($1.61) = $21.62/8 = $2.70/gallon.

CURRENT:
Now, it's no longer that simple.

It's still $18.40/case + 8.75% tax for one case, but if you buy two cases,
the price has changed to "buy 6 gallons get 2 free", whose math turns out
to then be 6 gallons x $4.60 each gallon + $0.41 tax each gallon = $30.00,
plus tax on the two free gallons at $0.41 each, which turns out to = $30.82
for 8 gallons, or ~$3.85/gallon of 12% HASA liquid chlorine.

But, you always get a card for "buy 11, get 1 free" at the store, so,
you end up subtracting $4.60 from that on every second visit, so, on that
second visit, you end up subtracting $4.60 but adding $0.41 tax, which
nets in a second trip cost of $30.82 - $4.60 + $0.41 = $26.63, which
then comes to $3.32/gallon on that second trip.

If you average the first-trip cost of $3.85/gallon & the second-trip
cost of $3.32/gallon, which is about $3.59/gallon, overall.