This is a massive YMMV but I have been able to replicate in Chicago and San Antonio already today, so please take it easy on the TD's. This is obviously not available everywhere but if you have Prime Now in your area you might be able to replicate.
Right now, Prime Now has a handful of items that are "buy 3 get $3 off" most of which are a few dollars. But Lifewater which is $1.09 in Chicago and San Antonio is also on the list. There is no limit to the number of times you can take advantage of the deal in a given order, but there are limited quantities in some areas. I have attached a screenshot from my San Antonio order where I ordered 45 bottles at $49.05 and got 15 of the $3.00 credits in return. When that was combined with my $5.00 Prime Now reward for no rush shipping it made it free. I added a tip but technically that is optional. You can also add on a box of free Honey Nut Cheerios in some markets including Chicago. You need to get to $35.00 for free delivery but that amount is prior to the promo codes. So, for my Chicago Orders I did 12 bottles ($12.00 back), then a $5.19 box of Cheerios ($5.19 back). I also added two 12 packs of flavored water that were buy one get one free ($4.19 back) and a few bottles of pure leaf tee that was "spend $10 get $3 back). So, on the $40.00 order total, I got $27.28 back and then $5.00 off for the no rush shipping code making the total $7.62 plus tax and tip. Screenshots below.