Sometimes it's ok to let someone do something kind for you as a thanks.

If I'm Bill I don't ask and if Jill asks I say "you don't need to do anything. Of she presses, then it's likely she's not doing so out of duty but out of appreciation; in that case if I'm Bill I say "You are very kind Jill And it's certainly not expected. I helped you because I wanted to, not because I expected something in return. If you'd like to do a little something then that's up to you and I will be gracious and let you do that just as you've met me help you. But zero expectations."