Did you order the piston from Napa or the caliper. I am seeing this part number on their website SE4890S. Can you confirm if that is the part. The piston itself is 17.99 and the caliper with the steel piston is 33.
I ordered phenolic pistons from Rock Auto. I have misplaced the p/n for the metal pistons from NAPA. Online at NAPA they did not have them in stock. You should have no issues with phenolic, not unless you are hauling a heavy trailer or some other activity that the brakes could get extremely overheated.