My main suggestions come from trying to use the search during the semi-annual sales, where IMS (and I could be wrong) the various coupons each have to be searched individually - i.e. the FREE group is not included in the 75%-off group, is not included in the 50% off group, etc. If my memory is wrong, you can skip the rest of this post

!
So for example on the normal search, where you can search FREE, or $0.99 or less, or $2.99 or less, and each group presumably picks up the others that are included within it, maybe there could be something similar for the coupons: FREE (i.e. 100% off), 75% or more off, 50% or more off, etc.
I understand that this doesn't say much about the actual price, since the starting base prices can vary. But it does come into play for me a little bit with series books, where if the first book is free, but the rest aren't discounted, I might think a little more about starting that series than starting one where the first book is free, the second and third are 50% off, and the rest are 25% off, or something like that.
JMTCW...