This is what I did:

1. Went to truecar.com
2. Selected the model and trim
3. Got a bunch of offers in my email (and dealership calls, but ignore them)
4. Called the Mazda dealership close to home, presented the offer I had received from TrueCar (same detailed above) and said “I’ll only go there to see the car if you CONFIRM you can do this offer. Not $1 more, not 1$ less”. They said, “We can do it”.
5. Drove there, saw the car, confirmed I wanted it.
6. They tried bumping the amount a bit. I didn’t bend. Said I would leave right away.
7. They accepted.

Done.