Fascinating. When Amazon was running its $30/$99 diaper promotions, where they gave Amazon gift certificates a month or so following a large order of baby products, people placed an average of 5-8 orders/day.
[I'm guessing here. Amazon reports what products get purchased through DT links and how much commission it generates--around 5%, btw--but doesn't reveal any info about orders or people at all. So to estimate the number of orders, I just eyeballed the daily Pampers tally and divided by $100 or so.]
Anyway, when Amazon launched their new Amazon Grocery offering last month by offering an immediate $10 discount on a $49 order [including diapers and a bunch of other stuff sold in cases], diaper orders dropped significantly. In July, diaper-related orders dropped to less than 1/day, while grocery-lookin' orders didn't show up at all.
The takeaways, then:
So actually, factoring in the different minumum order ($49 vs $99) and assuming the optimum % discount (20% vs 30%), it would appear that diaper buyers aren't really motivated by price discounts on a percentage basis [comparing $10 saved to, say, the $37 price of a case of Pampers] so much as on a perceived dollar value basis.
Or maybe optimizing that $10 discount by placing multiple orders as close to $49 as possible just isn't worth it: Math is hard! Let's not go shopping!