Welcome to Cruise Mates,
I am not aware of any "must buy" items, unless you are perhaps interested in native crafts. It is a world marketplace any more. You will be told there are jewelry bargains galore but they are not really that great. You might save a couple of bucks on liquor.
I prefer to get out of the shops and experience your new surroundings.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015