I always liked "I'm sorry, that doesn't work for me."
You can always say "I've done some thinking, and I've decided that it would be best if we each did our own (thus-and-such). We both need to start doing what's in our own best interests, considering."
Or something similar.
If she says "What are you saying, you can't do a simple thing like buy my cigarettes for me?!" say "Yes, that's what I'm saying. I think it would be best."
Value-neutral statements like "would be best" or "would be wise" or "probably best that we each", etc.
