First, congrats!!
Second, If you want to go, tell them you will. If the financial package isn’t what you need, you can always change your mind. The admission decision is essentially letting them know you won’t consider any other schools and are committed to RISD. This being said, if the financial package is awful, nothing is physically binding you to RISD. Kindly reject, rescind your decision if nothing can be negotiated, and if you’ve had other offers, start considering those instead! If they send out aid packages in early March you’ll have plenty of time to still decide and change course if needed.
Also, this doesn’t seem common in my experience, and I would also feel uncomfortable not knowing what I’m financially signing up for before committing. It really puts you in an awkward situation if things are not what you expected/need funding wise. Sadly, negotiating and navigating these things is part of the process!