Users can buy refill card at retail and add credit to the pre-paid account.

用户可以购买充值卡，给预付费账号充值。

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If a card dips below a minimum dollar value, Starbucks will automatically refill it from your credit card or debit bank account.

FORBES: The Overachievers: Starbucks Delivers

Google will also offer its own prepaid card, but it requires a credit card to setup and refill.

CNN: Mark Milian

Adding to monthly balances is accomplished by buying Refill cards or by registering a credit or debit card or PayPal account online at CommonCentsMobile.com.

ENGADGET: Common Cents Mobile becomes Sprint's latest prepaid brand, exclusive to Walmart