NO no no. The folks trying to sell you exam stuff with refundability are trying to get you out of your money.
I once bought a book to study fora postal exam because I was udner the impression that it was legit, and that it was necessary for me. I was wrong. I couldn't get a refund because it required that I take the exam something like 2 or 3 times and fail each time etc. etc. blah blah blah.
I lost out $50. I don't know where you would take the exam, or what it would cost you, but I seriously doubt $120. Check with the Postal Service Directly, or speak with your local employment office. They'll have legit info.