I actually asked my T about this, and she said, well, its a card and thats totally okay, but if there jad been money in it, I wpuld have handed that back to you.
I think once boundaries and rules turn into unnatural behavior, its gets difficult. A little human spirit should remain. I have had very professional and distanced Ts in the past (different country, too), and I alqays felt like there was are missing. Now I know its the warmth and human touch my current T shows.
I express thanks in a written way, and being able to hand her a card just helps me say what I cant put in words.

I see the issues with presents... But a card?