Ferry discount with camping cheques - How?

Quasar524

There have been mentions in various threads about about how buying camping cheques can lead to a discounted ferry crossing.  I have just looked online on the Club website at several possible crossings for next year, and the ferry price always comes out the same whether or not I include any CCs.

So my question is how do you find the crossings that give discounts with CCs?  Or can you only get the discounts via telephone bookings?