Amazon does indeed offer free shipping inside the USA. There is actually two different ways to get free shipping on your Amazon.com purchases and none of them involve some special coupon or magic shipping codes.
Both free shipping options are available to everyone.
***Method 1 - Amazon Free Shipping
If you select over $35 of eligible items excluding gift-wrap and taxes.
It is important to understand that eligible items will display "eligible for FREE Shipping" next to their price. You then need to ship all your items to a single address and select Free Shipping as the shipping speed. Items must be sold or fulfilled by Amazon to qualify toward reaching the $35 minimum purchase for free shipping by Amazon. Certain oversize and heavy items aren't eligible.
Find out more details on Amazon Free Shipping at:-
http://www.amazon.com/gp/help/customer/display.html/ref=prime_lp_help_page?ie=UTF8&nodeId=200337010&tag=yah-serv-20
***Method 2 - Prime Free Shipping
If you are a member of Amazon Prime then you automatically get Free Two Day shipping on most items.
Prime is amazing value that includes a range of free services ontop of the Free 2-day shipping including Prime Instant Video, Prime Music, Prime Photos, Kindle Owners Lending library.
They even have a free trial 30 day trial. check out more details here:-
http://www.amazon.com/Amazon-Prime-One-Year-Membership/dp/B00DBYBNEE?tag=yah-serv-20
I hope that helps explain Amazon's free shipping offerings.