Thank you note.

I had a phone interview last week from a recruiter in HR, an in person interview with the nurse manager this past Friday, and I have a shadow experience next week. When would it be appropriate to send thank you notes?

Also, I have store bought thank you cards. They are just blank cards that say Thank you on the front. Would these be appropriate to use, or is regular stationary more appropriate? Any advice is welcome!
I sent a thank you note after my shadow experience letting them know how the experience was for me and showing my enthusiasm for the opportunity to join there team. Congrats, and goodluck!
......store bought with simply "Thank you" on the front and a written note inside is what I did