The Mercenary set of items includes clothing and weapon skins, ship items as well as a compass.
Most clothing and weapon skins can be bought from any outpost.
The Blunderbuss and Rifle are unlocked by completing Mercenary Voyage commendations. Please see each item's page.
The Mercenary Ship items are no longer available. They were rewards from the Loyal to the Coin commendation.
Thanks to "Captain Kravos" for screenshots of some items.