I'm immensely grateful for the online courses on theoretical physics provided by the Perimeter Institute for free. A particular professor stands out for me, as the lectures were excellent, and the course was structured very well. Would it be appropriate or acceptable to send a small token of appreciation to the professor via (physical) mail?
I deliver a significant number of online courses, though not for university credit. I appreciate emails, tweets, and other expressions of gratitude from those who enjoyed them. I am not sure how I would feel if a gift basket or a box of chocolates arrived at my office. Sure, everyone likes gifts, but it might feel a little creepy to be honest.
I provide my courses to the organizations, and they're the ones who decide to make them free (Channel9 or MVA) or very cheap (Pluralsight), not me. So they are the ones who probably should be the target of the gratitude. Since PI wants donations, why not make one, then email the lecturer saying thank you and letting them know you've made a donation because of how helpful they were. (You could also repeat the statement in public such as tweeting it or writing it on the institute's wall, so that PI are aware of the lecturer's contribution to their coffers.) I would appreciate that more than chocolate, myself - and I like chocolate a lot :-)