Mike Snider, USA TODAYWalmart today is dropping the price of its iPhone 5s and iPhone 5c models to $99 and $29 from $149 and $49. Two-year activation is required for AT&T, Sprint or Verizon and each phone has 16 GB of memory.The discounts are only available in Walmart stores.Meanwhile, Costco earlier this week began selling iPhones online at discounted prices, with 16GB iPhone...