Supplier, College Grad, Credit Union, and Military discounts can NOT be combined. All of these discounts are the exact same discount called Preferred Pricing.
If you are wanting to use Preferred Pricing go here and build your car first.
http://prod3.gmautobuilder.com/Build...riceformula=SU
Remember if you decide to use Preferred Pricing, you can NOT negotiate below this price (you can not walk into a dealership and negotiate Preferred Pricing - $500) unless you qualify for USAA discount. This is a set price set by GM and any dealership who uses a Preferred Pricing code but sells the car for less can lose their franchise contract. The good thing about this is that Preferred Pricing is between $1,000 below to $500 above Invoice Pricing depending on your options.