Amazon.com is celebrating its 20th anniversary tomorrow. It has dubbed July 15 2015, ‘Prime Day’. You have probably heard of Amazon Prime. It works like this. For $99 per year, Prime subscribers are entitled to free two-day shipping within the US. Amazon started Prime in 2005 and has been adding in other benefits since then, like early access to lightning deals. Prime Day is the latest in the series of benefits Amazon gives to Prime customers. Amazon has stated that…