With Skull & Shackles, there will be a retail promo shipped out with every month's releases (base set and add-on in August and adventures 2-6 each following month). In addition to that, there are three other promos released/to be released so far in other ways.
As of the last PACG blog post, the following have been announced:
Ranzak - June 2014 (Free RPG Day)
Mogmurch - July 2014 (retailer packet)
Goblin Keelhaulin' - August 2014 (Gen Con)
Goblin Weidling - August 2014 (retail promo)
Owlbeartross - September 2014 (retail promo)
Goblin Pegleg - October 2014 (retail promo)
Magpie Princess - November 2014 (retail promo)
Mistmourn - December 2014 (retail promo)
Goblin Buckler Gun - January 2015 (retail promo)
Then there are three more for S&S that they haven't talked about yet.