Google Now gives users the option of purchasing credits directly through the Google Play Store. Having credit will make it much easier to purchase your next app or game directly on your phone or tablet without the hassle of finding a method to pay for them.You can purchase credits on the Play Store for $5, $10, $15, $25 and $50 based on your requirement.
To purchase credits for Google Play head over to the website and scroll down to the bottom of the page. Here you will find a link which says ‘Buy Google Play credit’. Click the link and you will be walked through a fairly simple checkout process
[source] – Google Play Store