Google payment options are divided into two types: Postpay and Prepay. One or both will be available to you depending on your country and currency. Note that you won't be able to switch from a postpay option to a prepay option or vice versa, so be careful when selecting your payment method as you set up your account.
If you select a postpay option, you'll make payments only after you accrue advertising costs. You'll be automatically billed either 30 days after your last payment or when your account costs reach your billing threshold, whichever comes first.
If you select a prepay option, you'll pay for advertising in advance of receiving any clicks or impressions. You can add funds to your AdWords account whenever you like, and we'll deduct costs from that balance. We'll notify you when your balance is running low. If you do run out of funds, your ads will stop showing until you make another payment.