link to selling on amazon tips :


" There are three ways to win the buy box every time:

Be the lowest price including shipping. If a seller’s account is in good standing and they are the lowest price including shipping, their product will go in the buy box ahead of other sellers. Note however, if a seller is in FBA, then Amazon assumes their shipping cost to be zero. The reason for this is that Amazon Prime members get free 2-Day shipping on all products and all other Amazon buyers get free standard shipping on any item or shipment that totals over $35.  "



http://www.skipmcgrath.com/articles/amazon-selling-tips.shtml