Last summer she wrote me a letter asking to meet. I replied that I would like to but she never followed up. ETA: I've sent her gifts every year. The first 12 years were books I liked at her age. When she was 13 I gave her a Tiffany charm bracelet and each year I add another charm for her birthday.
I love this. I wish my son's birth mom had some desire for contact with him. We know how to message each other through facebook and email, but she rarely ever reaches out. When she does, it's just something like a quick "Happy Mother's Day" type message. I worry that when DS is older he might have some feelings of abandonment. I think it's really awesome that you have found a way to show your bio daughter that you love her and haven't forgotten her.
I hope you continue to send something every year past age 18. I love that you do this and I look forward every year to see that year's charm.
Once she is an adult continued contact will be up to her. A) because I want to respect her wishes B) we go through the adoption agency to pass things on and the won't do it after this year