Most have encountered a problem purchasing directly with their credit card which results to an incomplete transaction. This method uses your credit card to load up on Google Credit without having to run to a 7 eleven store to get gift cards.
Step 1: Log in to your Google Account and go to Play
Step 2: Scroll down to the page and click Buy Google Play Credit
Step 3: Choose your credit denomination.
Step 4: Enter your credit card details and you will receive your Google Play Credit right away.
Step 5: Since the credit is already redeemed and into your account, directly launch ROVU and click Buy Rubies and complete purchase just like when you have gift card credit on your account.
Note: This guide is for those who aren't aware of this method yet.