Dawn Foods is a wholesale distributor. Given your unknown location, that might mean that you are required to provide a state/provincial business tax number. It might also mean that there is a minimum order.
If you are not a business, you should check out Costco prices. Chains like BulkBarn will also be competitive and you can ask for full bags of mix. You will need a professional scale if you are going to buy mixes in bulk. There just isn't any other way to measure ingredients consistently.
I personally use Duncan Hines which does come in bulk. It bakes up consistently the cakes are easy to handle. People have been satisfied in my experience.
There are a few caveats to buying large bags. DO NOT buy Domino powdered sugar in 25lb bags--it is 6X NOT 10X, and the same goes for some house brands that are packed by Domino.