How do I turn off account metering notifications?

I have just set up a new AT&T GoPhone (prepaid/pay-as-you-go) account with a Samsung handset. Each time a call is disconnected, a system notification pops up, informing me of both how much the call just cost, and the total dollar balance remaining on the prepaid account.

I do NOT want this popup to display. I've searched both the online account management, the phone navigation, and the user manual, and can't find a way to turn it off. Does anyone know how? The phone is a gift for my grandma and I don't want her stressing about how much her calls cost.