Changing contract terms

Every time we travel with MSC they hike the prices and change the rules. It has definately gone downhill. I have confirmation of an upcoming cruise and it used to be 60 days payment in full , now they have changed it to 85 days . How can you change the payment terms half way through a contract ?.. Also at the time of booking the R3000 deposit was refundable...now it is not . I think this is ********. I wont be using them again, enough MSC.