Should I wait to have my kittens ear mites treated?

I have 2 12 week old kittens, and I looked in their ears today and noticed that they have a dark, waxy buildup. I haven't noticed any other symptoms like scratching or shaking their heads. They have an appointment to get fixed in a little over a month, and I was considering waiting until that appointment to have their ears checked (I really don't have the cash to make a whole separate appointment for this). I am scared to try an over the counter medication because I don't want to damage their ears. Am I putting them at risk by waiting a month?