"So if you put in an extra $3,000 on Aug. 3rd, another $3,000 on the 15th and another $3,000 on the 29th, then my understanding is that you would taxed at 1% of $9,000 for the month. Then in September you purchase another excess $10,000 and you're taxed at 1% of $19,000. I would think has established a limit, but I cannot find it anywhere on their site."

Another interpretation is that if you over contributed by 10K and it went down to 8K owing to market activity, you would still pay on the 10K... the highest amount.