Shares of Amazon went up like a rocket in after-market trading Thursday following the company’s unexpectedly strong first quarter earnings report. And they kept going following the opening bell Friday. As of mid-morning trading the shares were up more than 12 percent, as investors applauded Amazon’s biggest year-on-year increase in operating profit in 10 years. Much of that uptick can be attributed to the increasing share of Amazon’s sales coming from digital products, including e-books, movies, music and apps, which cost nothing to ship. The company saw a 19 percent increase in media sales in the quarter and nine of the top 10 selling items at Amazon.com are now digital. The increase in digital sales is also a testament to the success so far of Amazon’s Kindle Fire strategy. According to the latest comScore data, the Kindle Fire has captured more than half the Android tablet market.