So, now the question is...what should I do with this additional income ($600-900/month). We (husband and I) have no debt except the mortgage and we are already throwing money ($2000/month) into index funds.
...and? There's no limit to the amount you can save. ;)
I started doing this to get experience in something that I want to do later, so the money is really a bonus for me.
Sure, it's unexpected, but it's still money that can be put to use. There's no rational reason to blow it, or use it any differently than you are with excess money (that 2k/mo) you are now.
Is is silly that I want to treat this money differently from my normal income since it's "extra work"?
To put it simply... yes. :)
Some people want to be a little silly, and that's fine. Others prefer a rational approach, and that's okay, too.
Decide what you want, what your goals are, and go for them. Then something like this won't even be a question, because your spending will align with your values.
Good luck! :)