Yes, they can be used at the airport, but last year, I had one voucher that kept generating an error message when the Frontier agent tried to enter it. Another agent tried and also got the same message. As a result, I used it online and paid the CIC (Carrier Interface Charge). (Customer Service adamantly refused to reissue it saying that it was a working voucher.) A few days later, I was about to write the DOT a complaint letter, and I received an e-mail that the nonstop flight that I had reserved was being replaced with a connecting flight with a 5-hour layover in ATL! I demanded a refund and a new voucher. Today, I redeemed the $50 voucher at the airport, and it worked AND covered taxes and fees. Some vouchers don't, so you have to read the Terms and Conditions on the voucher.