Many (not all) of those coupons come from the "Entertainment Book." And basically, for the most part, the small print says that the coupon can only be applied to certain fares (which are normally the regular priced ones with no discounts or promotions). In my experience, you can normally find a fare through TA's or online websites that come out cheaper than if you used the coupon with the indicated fare. In any case, if you want one, I think I have some at home and would be more than happy to mail one to you (this way you don't have to purchase it on ebay). If you're interested, email me at firstname.lastname@example.org...and
let me know which cruise line you're looking into and I'll see if I have one for it.