Personally, I wouldn't offer free fixes on anything I didn't write entirely by myself. And I would only do it if there were sufficient scope and flexibility for me to include all my quality-oriented practices (unit testing, pair programming, good logging, incremental deployment, etc). So in the future, I think my answer is, "Don't do that."

But that's the future. For now, you made an offer. I'd honor it for the current list, and then send them a note saying, in fancier language, "Hey, I've spent 8 hours running down issues that turned out to not be my bugs. I am happy to stand behind my work, but I can't afford to do that for other people's code. If you folks would like a support contract for your system, I'm glad to do that at $XXX per hour with a monthly retainer of $YYYY. I'm still glad to fix any bugs in my code for free, but in the future, time spent on issues that aren't caused by a bug in my code will be billed at my normal rate, $ZZZ per hour with a 1-hour minimum per incident."

Most clients are great, but few think through the economics of freelance software development. And some people are just greedy. The best thing I even learned about client pricing was to structure deals so that no matter what happened, I was ok with the outcome.