3.3 billion is today's daily digit. That's the valuation Britain's Royal Mail is attracting after its share offer was announced. As a public institution, it's nearly 500 years old. As a listed company, it'll make its stock market debut in October. Over 40 percent of shares will be up for grabs - at 260 to 330 pence. The public are expected to snap up just under a third of the stock, though they'll have to invest a minimum of 750.