I see a charge on my bill that I do not recognize

NetZero may not have been able to collect your monthly payment in the previous month/s (Click here for reasons as to why this could have happened). Hence, you were charged the balance as well as the current month's charges on your last billing date.

Or,

You may have upgraded your service to NetZero HiSpeed or added another NetZero service, such as MegaMail or our Family Pack. When you upgraded, you may have incurred pro-rated charges, which carried forward to your next billing date. Hence, you were charged an amount that is different from the advertised rate. However, please note that going forward, you will be charged as per the advertised rate.