This is what I would tell my daughter:
You sound ungrateful, not because you are not sending thank you notes, but because you are complaining about such a small thing. BE GRATEFUL THAT YOU HAVE A GRANDMOTHER THAT IS IN YOUR LIFE!
All it takes to make her happy is a little note in the mail? That's not much. That's a teeny little thing. Just do it and feel glad that you can make her happy. She comes from an era when people did that--she loves mail more than email and phone calls. But you're going to dig in your heels because you refuse to be a gracious person?
There are tons of people who would love to have a grandma who cares, give gifts, and smiles at you when you do nice things. You should smile and feel loved as you shop for a box of sweet little cards, then send one off in the mail each time she does anything. Be creative and fun. Make it be a special grandma thing that you will tell your kids about. You'll like yourself better too.
I can almost promise you that she will keep the cards and think fondly of her one grandchild that is not a self-centered, roll-your-eyes-at-me teenager. Someday you will have the cards back and treasure them because they made your grandmother easier to love.
ETA: I posted when Rusty did. Ditto.