We have an honorary grandma (mother of a friend) who sends my kids pretty lavish gifts on EVERY holiday and birthday. I've been sort of beleaguered in the past by the mountains of thank you notes I always have to write, and since my kids get tons of stuff from tons of relatives, I sort of wondered why she did it. BUT, I always made sure to write a thoughtful card, usually inclusive of photos including the gifts, etc. I felt, she took so much time, and spent $, it was the least I should do, despite sort of wishing she wouldn't send stuff due to my own disorganized, cluttery, overscheduled personality.
Well, I heard through the grapevine, that her own kids never send thank yous for the things she sends to their kids, they always get lost and forgotten, etc. and for that reason, my kids are sort of "special" to her. Also, she has a very tough time caring for her husband's advanced Alzheimer's and some other issues, and it means so much to her that we appreciate the gifts and consider her a grandma. So ANYWAY, just thank her, she loves you guys, and it does her heart good to have you being appreciative.