With the amount of times autopay seems to fail. Adding the funds a couple days before your payment is due seems to be The way to go. Lol.
@jerryxue1600 my plan was canceled the day of renewal simply by not paying. I bought a voucher and my phone was up and running as soon as I called 611 and entered the code. You likely will not have any issues if you simply don't pay. Like others advice, check autopay is not active. Go to the payment tab, select make payment, go to manage autopay. From there you can double check your card is not still entered for autopay.