How does the fate of Mrs.Crater, Lucynell, and Mr.Shiftlet connect to the title of "The Life You Save May Be Your Own"?
O'Connor's religious vision deals with humankind's fall from grace. Her comedy shows her characters' blindness and inability to redeem themselves through nihilism and materialism. The title "The Life you Save May be Your Own" deals not with highway safety, but with salvation in the afterlife.
Mr. Shiflet's fate is just as his name suggests: a shiftless wanderer. He begins and ends the story lost. Though he has the ability to redeem himself by marrying an Innocent, Lucynell, he abandons her in favor of a car, a few bucks, and a hitchhiker. Ironically, the hitchhiker boy abandons him the same way Shiftlet abandons Lucynell.
Lucynell remains silent the entire story; as such, she is an Innocent, uncorrupted, the Virgin Mary, and "angel of Gawd." She is blind to the cruelties and materialism of the world. But, because of her mother's blindness, Lucynell is abandoned on the highway. In the end, we must assume that she will be corrupted too. As she is literally lost by the end of the story by Shiftlet, her life too is not saved.
Mrs. Crater too loses everything in the story: her daughter, her car, and her salvation too. She has been deceived by an imposter, Shiftlet. Her faith in him is misguided; instead of protecting her daughter, an Innocent, she ends up brokering a marriage to a fraud, paying Shiftlet--a kind of pagan, unholy dowry.