Cancel Flight Ticket - Allegiant Airlines Cancellation Policy
In this case, when you are not eligible for a full refund, you will receive a voucher that you can use on future Allegiant Air bookings. When booking your Allegiant ticket, you should also keep in mind that you will not be able to make changes to your travel itinerary within 7 days of departure. If you are a Trip Flex member, you need to complete all changes one hour before your scheduled flight departure. As a Trip Flex member, you can change or cancel your flight reservation up to 1 hour after your departure date without paying any fees.
https://www.cancellation-policy.com/allegiant-airlines-cancellation-policy