But there's a first time for everything. First, some background. There’s a new tech start-up called Jet.com. Maybe you saw it on the cover of Bloomberg Businessweek. It's a membership-only online shopping club, similar to Costco, that says it will be able to charge between 10 and 15 percent less than other e-commerce sites by, essentially, cutting its margins to nothing. Jet hasn't launched to the