-- WAIT! --
Don't fall into the trap of focusing on your
IP. It's very simple to work around your patent or IP and for the company to cut you out of any future royalties (or at a minimum put legal challenges and expenses in front of you). The company may even decide to shift directions and move away from production of this product for whatever reason. Then you are out with nothing.
Approach this relationship from a broader perspective. Trust me, once they get into production or packaging they are going to think that your original patent doesn't really cover the new design. They are going to file new patents, etc, etc. And your "value" will be discounted greatly.
Cover your bases here.
[Happy to discuss. I've filed patents, competed against patents, and have friends who are designers that have done exactly what you are thinking of doing.]