are you a GM rewards member (should be free) - buying a vehicle gets you a substantial number of "points" that can be used to buy accessories/parts or use towards a vehicle. I think there is usually a delay in the points being awarded so they may not be able to be used immediately, but it's an option for anything needed later.
Any decent set of mats will be a couple hundred, and unless you find them on FB, I've never seen 40-50% off deals
We have purchased a couple sets of the GM all weather mats and usually you can get a discount if you purchase online vs walking up to the parts counter. right now online has a 20-25% discount. I went to the dealership several years ago and I joked with the parts guy about a getting a discount, he said buy them online and set the pick up for there...I stepped away from the counter, purchased them on GM's website and then he processed the order. Granted I probably only save $50-75, but that's more than nothing. Have to make sure they have them in stock first if you want them same day...just a thought.