I do it already.... I chase discounts all the time for parts.
And the dealers and suppliers that give me the best prices get my business.
I needed keys encoded for my Mustang, I don't have my high end scan tool here so I had to pay someone to do it.
The local dealer wanted $145 dollars, plus the cost of the key $39.00, PLUS sales tax and 3%
I ordered the keys online and a local locksmith did it in 15 minutes including cutting 2 keys and going into the computer to add the keys.
I gave him $60.00 cash, no receipt needed
Why did I walk out of the dealership? Not the cost but the 3% add on.... and I could have easily paid cash.
I wanted new wheels for the car, I could have gotten them locally for 140 each, there was a dealership 90 minutes up the road that was willing to do it for $95 each and take my Florida resale tax number saving me another 25 bucks in sales tax. To save $200 bucks I'll take the trip as I didn't have anything else to do.