JetBlue Airways Corp. /zigman2/quotes/207639051/composite JBLU +1.93% upped its proposal to buy Spirit Airlines Inc. /zigman2/quotes/205782179/composite SAVE +0.65% again, trying to outdo Frontier Group Holdings Inc. /zigman2/quotes/211317788/composite ULCC +0.49% , which sweetened its own offer for the ultra low-cost airline late Friday. JetBlue raised the fee it would have to pay Spirit if the deal wouldn't clear antitrust to $400 million, from $350 million, and its prepayment offer to $2.50 a share, payable as a cash dividend to Spirit shareholders "promptly following" a shareholder vote approving a merger with JetBlue. JetBue also added a "ticking fee," which would provide Spirit shareholders with a monthly prepayment of 10 cents a share between January 2023 and the closing or termination of the deal, expected to bring close scrutiny from antitrust regulators. JetBlue called its newest offer a "decisively superior proposal," and urged Spirit shareholders to vote against Frontier's offer. Frontier's offer recently got the recommendation from proxy adviser ISS, which was a change from ISS's previous recommendation, and has secured a recommendation from advisers at Glass Lewis. Spirit shares rose more than 2% in the extended session, after ending the regular trading day down 8%, while shares of Frontier rose 1% after an 11% loss during the regular session. JetBlue shares ticked 0.6% higher in after hours after rising 1.6% in the trading day. Spirit shareholders are scheduled to vote on Thursday.