57.72 does it for me--it is hard to beat $19.24 per book. Unfortunately they wouldn't let me re-sign up with Amazon Prime under a different log-in (must recognize my credit card name and address, even though I used a different card).
I am doing super saver shipping, which takes a bit longer, but A) As someone mentioned in a different thread, why do we need instant gratification?, and B) The main difference is when they ship: regular shipping goes out on June 6th, super saver on June 10th. I don't think it is actually a different method. Now given that the 6th is a Friday, I probably wouldn't get it until Monday the 9th or the 10th (MAYBE Saturday the 7th, if I was really lucky), whereas I've noticed that I almost always get stuff via super saver a day or two after the ship date. So we're talking the difference of a couple days, really. I'd rather keep the $4 ;-)