If you haven’t sent out your Christmas cards yet, you might want to get on it, quick. Snail mail just got slower. The cash-strapped U.S. Postal Service announced Monday that it’s going to end next-day delivery of letters, postcards and other First Class mail. The agency is looking to find $20 billion in annual savings by 2015, about $3 billion of which could come from various plans to shrink the network. But what will become of the letter? Lisa Grunwald is author of the novel “Irresistible Henry House” as well as two books of letters, including the epistolary history “Letters of the Century: America 1900-1999.” She talks about the demise of the letter.