Some of them rely on users to submit ones they used. Think of it as an aggregator of shared coupons.
Sometimes these codes have quantities and so they run out of uses. Others are specific to the user who submitted them and are single use. Some are tied to a region and then others, of course, have since expired.
Coupons are meant to be targeted marketing. When you try to broaden their use beyond their intended target, they aren’t as reliable.
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