You can only redeem your rewards for Starbucks menu items, and only after you accumulate enough Stars. If you fail to spend earned rewards, they will expire as fast as in six months. If you want to get more freedom, consider applying for a general cash back card that is not tied to any particular brand. Every cash back card allows you to redeem rewards towards anything you want. There are cash back credit cards that do not have an annual fee (unlike rather high $49 Starbucks card annual fee) and even have 0% intro APR.
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All rates and fees, and other terms and conditions of the products mentioned in this article/post are actual as of the last update date but are subject to change. See the current products' Terms & Conditions on the issuing banks' websites.