If you are an independant contractor, and not an employee of the store, then you should pocket all of the profits.
They asked you to teach there. That means that they are providing you with a facility, and student base. You in turn are providing them with customers who are going to buy all the stuff you tell them to, like hearing protection, ammo, etc. The store profits from that. Let them know how you are advertising classes at their store and how that brings in additional customers to them.
NRA Certified Firearms Instructor
Utah Concealed Firearms Certified Instructor
SABRE Pepper Spray CSAP Instructor