Quick answer
Where Should You Buy Souvenirs Before or After an Antarctica Cruise?
Ushuaia is usually a better souvenir stop than the ship if you want variety. Buy gifts, postcards, patches, or local items before embarkation or after return, because Antarctica landings are wildlife and wilderness visits, not shopping stops.
Before departure
Leave room in your luggage and check customs rules for any food or natural-material souvenir. Buy only items you can lawfully take home.