The Postal Service launched a new Web site today to sell its vision for 5-day mail delivery (which lawmakers may be slowly warming to). Many of you probably know the highlights already: USPS thinks 5-day will save $3 billion per year, post offices and other facilities will remain open on Saturdays, etc. One thing that’s not posted yet, which I know is of great interest to the Postal Regulatory Commission, the Government Accountability Office, and other analysts: An official estimate of how much mail volume will suffer from switching to 5-day. As I said on Monday, I’m skeptical of the…