How to respond to a rude bug report?

I just received a quite rude bug report. The user basically says that we're doing it all wrong using capital letters here and there, although he's in fact just pointing at one bug.

On one hand, I care a lot about our users and want to maintain a good relationship and a good rating of our app. On the other I'd feel like a complete sell out if I would reply overly polite.

What's a decent way of responding? What should I keep in mind? What mindset should I have?

^{It should be added that the user seems to be a 24 year old CS student, and our product is an Android app that we're giving away free of charge.}