Free shipping is known to be one of the most effective marketing tools e-commerce sites have.
Shipping fees can affect the number of items a customer purchases, and they can decrease how frequently a customer makes purchases from the site:
With fees, shoppers will make fewer shopping trips and purchase more goods at a time -- not unlike shoppers who drive great distances to a particular store, Bell says, and decide they had better stock up while they're there. Alternately, fees can prompt consumers to simply walk away. A survey from 2004 found that shipping and handling costs triggered 52% of the abandonment of online shopping carts, Bell says.
Studies have shown that while many customers will "browse" your products, a very small number will actually place them in the cart. If you do not put the free shipping option on the pages that are being browsed, then you are only showing one of your most compelling sales motivators to as little as 19% of your visitors:
In another study for a major department store retail chain it was determined that there are two main reasons for cart abandonment: (1) Customers cannot find what they are looking for, or (2) they do not care for the shipping options. Our study found that 56% of all shoppers browsed for products, but only 19% placed an item in their cart. The most cited reason for this first level of abandonment was, “I can’t find what I’m looking for.” Of this 19% only 7% of shoppers completed their transaction. The most cited reason for this second level of abandonment was, “the shipping costs and options were unacceptable.”
As such, I would not treat it as a transactional element but as an enticement to get the user to commit to a "buy" decision.
The best place to put this information is directly next to the item's price. If you don't then the user is not given a very compelling reason to reach a "buy" decision, and will therefore never see that free shipping is available.