Thumb Score: 0

American and United both have plenty of domestic roundtrip flights pricing under $ 99.

Source: List of routes below taken from a Flight Baron email alert

Travel Window: Varies depending on route. Some have more availability than others. Now thru May 2018 is search range.

How To: Use Google Flights [goo.gl] to search and use their Calendar feature to find the lowest priced fares. You can eliminate low-cost carriers from your search results by deselecting them in Google Flights. Alternatively, you can select United and American as the only results to show in your Google Flights search.