Story coupons are specific to one story and can only unlock an episode from that story. They cannot be used to unlock an episode with a lock symbol next to it -- those can only be unlocked using coins. Coupons cannot be purchased, they can only be gifted as part of a reading event, from Radish, or from the author.
To learn more about coupons, check out our guide here.
Coins can be used on any episode of any story!