Will an underseat bag fit on Spirit, Frontier, or Allegiant without a fee?

Only if it actually measures within 18x14x8 once packed. Specialist-subreddit threads repeatedly warn that budget carriers gate-check bags into the sizer, and verified-purchase reviewers note that some rolling underseaters from BAGSMART, LUGGEX, and LOVEVOOK exceed 18 inches in height once wheels and handle are counted. The Narwey 18x14x8 duffle and the Travelpro Maxlite 5 tote are the picks most frequently cited as compliant.