Open Road titles now discounted at Sony, and some have a tiny further 5% off additional discount, taking the originally $9.99 list priced ones to $3.80 instead of $4.
And for what it's worth, it looks like the Kobo coupon is for 60% off the already discounted price, taking the test-book I checked from $19.99 "list"/$12.39 "your price" to $4.96 before tax.
A hypothetical $9.99 list/$7.99 your price book could go down to $3.19 and
these Richie Tankersley Cusick horror novels, for example, go from $2.69 to a mere $1.08 (Amazon's price is $1.20).
So if you manage to find something you want that's not already cheaper at Amazon and Kobo doesn't do weird separate pricing shenanigans for your region, then they'll probably be the cheapest place to buy.