It is a free process. The only cost is the proper box to send for certification. They will send it back to you after the test if you check the proper box.
I was FedEx certified. My account got hacked, and $750 worth of charges showed up on my bill before I ever sent anything. They were good, and I didn't have to pay, but since the fraud department at FedEx had taken over my account, I would have to go through the whole process again, because that certification is not transferable between accounts.
I registered with ShipYourReptiles, and checked their prices; they are cheaper than FedEx, and I don't have to go through the whole thing again.