DALLAS (AP) — Shareholders of Spirit Airlines will vote next month on a proposed merger with Frontier Airlines. If that deal fails, Spirit could be sold to JetBlue Airways. A vote on the Frontier deal had been scheduled for Thursday but was delayed until July 8. The outcome could affect fares for millions of air travelers who depend on the budget airlines. Some industry experts think a Frontier-Spirit combination would keep fares lower than if JetBlue buys Spirit. Either way, consumers will probably find something to complain about. Spirit, Frontier and JetBlue had the highest complaint rates in the industry last year.