Answer:
In order to qualify for Amazon Free Shipping you must purchase at least $25 minimum of super saver eligible items. Items that qualify will indicate on the product page that they are eligible.
Often a purchase falls short of the minimum required so you may need an amazon filler item finder to find cheap filler items for free amazon shipping. You can use a tool such as this to fill in the gap amount so you don't end up spending more than you actually need just to get the free shipping.