You seem to indicate that you are a NRA Certified Instructor so I assume you are aware that you are encouraged by the NRA to have your own insurance that specifies you are covered.
Does this gun store have insurance coverage for this type of activity? They should be able to provide you a Certificate of Insurance if they have coverage.
First, I suggest consulting with your attorney and have him or her at least review your contract.
Who is paying for the class materials? Who owns the range and classroom where these courses will be taught? Will you be able to handle the number of classes and students this gun store will or may require? If you are going to charge a fee, you may want to look at Material Costs + Range Fees + a Flat fee + $X per student. The fee structure paid to you would be different for each class. Put everything in writing. I would suggest having an agreement on a Min and Max class size and that you have the right not to teach an individual if you so choose. I would also suggest having an exit clause so you know ahead of time what will be required to end this contract. It will cost some money, but I suggest consulting and working with your attorney.