Unfortunately after Royals final payment date there is no refunds or OBC offered. They did offer this during Covid but that stopped last year.
The only thing you can hope for is upgrading to a higher category that can be done for a low additional cost.
Note, It is Royals final payment date and not when you made final payment that is the deciding factor. If you are before Royals final payment date then you would get a refund and not OBC