We also just received our first bill.
Mine is due 9/1.
it says Past Due After 9/15. So yours would be past due around 8/31.
so it looks like if you call on Monday and pay by credit card, you should be oK.
on the back, it says any payments received after the Past Due After date will be subject to late fees in the amount of $25 per contract plus interest at the maximum rate permitted by applicable law accrued from the original payment due date. The avoid these late charges, please allow at least 7 days mailing time.