I was always taught to give thanks to the plant, and you are correct about gifting the plant. A bead, a shell or a pinch of tobacco are common gifts. We were also taught; as were you, not to pick all the growth from the plant, but leave enough for 7 generations in the future.
My brother and I are working on saving the native herbs and medicines in our area. A lot of good books and information about the farming practices in many areas are recorded by the tribes themselves, but usually are only passed down orally. Which means too much of our heritage has been lost. How do you explain what a pinch of something is, when they don't know how big your Grandma's fingers were? It isn't just a matter of going out and collecting, but there is also a spiritual connection between the collector, the plant, and the end user of the plant. It is all part of the great circle.
Keep up the good work.
Walk In Beauty!