To use a voucher or credit on Frontier Airlines, during booking on flyfrontier.com or the app, reach the payment screen and select "Credit with Frontier" or "Frontier Voucher." Enter the confirmation code or voucher number from your email. Vouchers are valid for 90 days from issuance and apply to airfare only, not taxes or fees. If value remains, it's retained for future use. For issues applying credits or vouchers, contact (844)734-0813. Note that vouchers are non-transferable and must match the passenger name; eCredits from cancellations work similarly.