Most likely, a web browser extension (such as AdBlock) is interfering with your attempted purchase. We would recommend disabling all web browser extensions for Firecracker, and/or trying another modern web browser (such as Safari or Firefox). 

On Chrome's newest version, there is a vertical stack of three dots in the upper-right; you can hit that and then go to 'More Tools' to manage your web browser's extensions.
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Firecracker accepts most major credit and debit cards, including MasterCard, Visa, American Express, Discover, and JCB.