When you're given the option to use a credit card saved to your Google account, it will only ask for your card's CVC to confirm. Of course, it will only work if you also sign in to your account on the other device you're using. You can also delete the cards under Payments & Subscriptions in your Google Account page if you'd rather not risk your CC details... or if you don't want to make it too easy to buy things wherever you go.