An additional column when selecting stores showing parts that have already been accounted for, but could potentially be fulfilled more affordably would be really helpful.
It seems like many of the stores with the largest inventories that I select first when starting to feel out the purchase set certain price-per-pieces noticeably higher than some smaller stores. I usually zero in on a significantly better price by selecting a few medium sized stores that supply the bulk of the parts I need for significantly less than a single or couple large stores do, then figuring out the easiest way to pick up the few straggler rare pieces with as few additional stores as possible. I've hit on some medium stores that when selected jump up the list fulfilling large chunks of parts more affordably than ones already accounted for and cutting the total price down noticeably even with the added shipping charges.
Being able to see the number of parts that could be cheaper than ones already accounted for would make that process much quicker and remove a lot of the guesswork.