Hi!

Interesting idea. We collect feature requests over on our UserVoice page. You might want to add this there as well.

While we don't offer that exact feature, we do have something similar that might work for you. We have billing alerts that you can set that will email you if your monthly spending exceeds an amount you define. Each time you add credit to your account, you could set that value to be slightly below what you've added. Check out the blog post announcing it for more details.