You can buy AT&T Prepaid (formerly GoPhone) minutes online through the official AT&T website or third-party retailers. Official AT&T Online Options
You can refill your own account or someone else's without logging in by using AT&T Quick Pay . You only need the 10-digit phone number and a valid payment method.
There is typically a $10 minimum for credit/debit card payments made directly through AT&T. Make an AT&T Prepaid Payment
Digital codes are available from stores like Staples .
Sites like Recharge.com and USA Prepaid allow you to buy minutes directly or send them to another person's number.
You can also purchase digital refill codes that are delivered via email from authorized retailers: