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Online promotional/coupon codes found outside of what's being shown on flipgive.com can void your cash back. All valid coupon codes will be featured in the yellow banner when you click a retailer's link.
Additionally, some items may be listed as 'excluded', meaning they won't earn cash back for your team. All this and more can be found on the 'To Qualify for Cash Back' section when you click on any brand (see below):