She probably does feel guilt and probably also wants to maintain contact with you. Give her the benefit of the doubt and assume that she doesn't mean to guilt-trip you but maybe is giving gifts in hopes they keep the lines of communications open.
Think about her side of it for a minute, because right now you're stuck in thinking about your side of it (costs money, she's giving me a guilt trip, I feel bad, I am being forced to reciprocate, I have past hurts....). She may feel that she did stop with the other things as requested -- no more Christmas gifts, no more gifts for each others' kids -- but she doesn't want to stop the birthday gift. "I can't compromise on that one because it's the most personal and it's about HER and not her kids or a holiday," may be how she's thinking. So her motives may be positive and an attempt to be thoughtful, not an attempt to get you to reciprocate. Just something to consider.
Just send her a very nice card with a friendly and non-committal note in it and don't feel obliged to send her a gift in return. She may want the contact with you much more than any gift. Do send her a birthday card because what's the harm in that, unless you really don't want any contact at all with her to continue? If you really do just want her out of you life entirely, tell her so, because it sounds like she may think there is more left of your friendship than you think is left, and it might be kinder to tell her the truth so she does not continue to reach out to you.