Let them by all means do the prep work. This gives them full range to not warrant any work because, like Mbrooks said in a way, if it peels or gives away, they will straight up blame it on your prep work and you will walk away empty handed in the end. You can buy paint from PaintScratch.com, but again, you want a shop to color match it, new paint mixed by the color code doesn't have age included. Shops should be able to grab a sample of your existing paint, get it made, and then spray, and buff/polish it to match.
I'm paying $550 this coming weekend to get my entire front bumper/airdam smoothed to glass, prep'd, painted, and clearcoated, as well my new grille shell (prep, paint, clear). It's not that much of an area, like you're tailgate, but it's quality work. Don't make the mistake I did and go cheap the first time, cause right now, I'm paying almost 300% more than what it costed the first time around (which is why I'm getting things reshot and redone). I won't have my front end for about a week - a full cure is about 3-5 days, which he wants to do, make sure you keep that in mind.