Hey D of B,
#1. I would price a rad out from a few parts stores first. Flip open the yellow pages and get a clean sheet of paper to write down the details of the quotes you ask for and the clerks name. You'll be surprized at the money you may save just on the rad.
#2. $270 for the work seems high. 4.5 hours at 60$/hour? I would price out a few other quotes as well. Call a shop with year , make and model,transmission with a/c or not and see what you are quoted. It only takes a few minutes to do.
#3. Check the back of your copy of the certifacation sheet and see if the rad falls under areas that have to be checked for a safety. It may not be part of the inspection. I guess a car can have a bum rad and pass the test and the tech. may not even notice.
#4. Make sure you are not billed for a new thermo. when the work is carried out. It would also be a good time to change the upper and lower rad hose and install fresh, properly mixed coolant. ( Honda coolant is premix'd $14 aprox. for 4 liters )If you use the Honda coolant make sure you point out that it's premix'd to the tech. I had a flush done 2 years ago and you guessed it they mixed the already mixed coolant from Honda that I supplied. The rad hose will not add to the repair bill much if at all.
Good luck