Check there isnt any dodgy terms and conditions with the offer in the very small print.

I know some local ferries do special return offers now and again usually on a sameday basis. Apparently if you dont complete the same day deal, they will then charge a full fare.
Although if you had a very good reason they might let it go.

Saying you missed the plane then just decided to stay on, probably isnt showing much intent to actually fulfil your end of the transaction.


Although on the other hand, they can cancel flights as they please leaving you in the shit, so fuck em lol.