Amazon mandates that they charge exactly $3.99 for S+H, because previously when everyone set their own charges for S+H, it was difficult for the consumer to com

Amazon mandates that they charge exactly $3.99 for S+H, because previously when everyone set their own charges for S+H, it was difficult for the consumer to compare prices from different vendors, and the vendors were incentivized to inflate S+H so that their books would look cheaper than they were. Of course it would be more logical for Amazon to mandate S+H of $0 so that what you see is what you pay, but I think the current system is a reasonable compromise.