When we went from LA to Buenos Aires a few years ago we were booked for the whole itinerary. Didn't realize it was also sold as segments, but it never interfered with our plans or excursions. Guests on and off for their leg were hardly noticeable.
If you go to the "Post Office at the End of the World" in Ushuaia and want to mail post cards drop it off inside. We put ours in the mailbox outside the entrance door and are still waiting! Maybe they never checked the box lol.
Also, Machu Picchu is an experience of a lifetime. If you're on the several day excursion the hotels where you stay and the restaurants serve coca tea at breakfast and coca candy, tea, energy bars are sold everywhere. Yes, coca as in cocaine but without the high. If you get altitude sickness (any many do) it resolves it very quickly. However, don't try to take it back on the ship or across the border because you'll be in arrested (they warn you). Not a good way to spend a vacation.
Can't wait to hear about your cooking class in Buenos Aires! We always heard about the beef in Argentina, and have to say it's the best we every had, Funny about the restaurants serving dinner so late. We stayed in Buenos Aires for 2 weeks when we disembarked there. First night asked the hotel concierge to book us at a restaurant for 7:30 pm. She laughed and told us no one eats dinner before 9pm and that was early. So we did as the locals do and it was great. Also, many different types of empanadas and all delicious!
Patagonia is a beautiful and breathtaking part of the world. Enjoy every second of your trip and safe travels!
Lorraine