There used to be a topic here for it (how I found out) but I can't find it now.
You have to have a single item worth more than $25 ships and is and SOLD by Amazon. When I used it I had to change the orders shipping speed afterwards since it automatically choose the standard shipping for me. That's it.
I thought some might have missed it.
(Offer expires at 11:59 p.m. (PT) June 30th, 2014.)
Not sure when (if ever) the promo credit itself expires though.