Spirit Airlines stops flying at 3 a.m. Saturday. The DOJ blocked the $3.8 billion JetBlue acquisition two years ago to preserve competition. Spirit pioneered ultra-low-cost flying in America. In 2007 they unbundled the base fare and sold the bag, the seat, the snack, and the boarding priority as separate line items. By 2017 ancillary revenue hit $53 per passenger, up from $5 in 2006. The math worked. In 2009 Spirit posted a 15.9% operating margin, the highest in the US airline industry. Then…