i'm sorry to be the lone voice if dissent here, but the ugly fact is the ugly fact.
you got the price you quoted.
you invited her to pay what she could afford.
the reality is that if she couldn't afford so much as a nickel at the time, you'd have been obligated to give it to her for free.
you learned a lesson worth waaaaaaaaaay more than $20.
when doing business, it's best for both parties to be clear and specific.