You can also pay your bill at the straightalk web site. You use a credit card in this case.
In my opinion, ST is a very inefficient and error-prone company. The service is at best acceptable, not great. But it is cheap, and for voice, text and VERY basic web surfing, it is OK.
The best way to renew your service is to buy a card from Wal Mart, and renew the service from the website.