Online retail giant, Amazon Inc. (NASDAQ:AMZN) has begun testing a new subscription model for unlimited grocery delivery to its Prime members at an additional cost of $9.99 per month. What Happened: The initiative allows Prime members in three U.S. cities, Denver in Colorado, Sacramento in California and Columbus in Ohio to avail unlimited delivery on Amazon Fresh and Whole Foods orders of more than $35, reported The Verge. The subscription cost is added to the basic Amazon Prime subscription fee of $14.99 per month. Moreover, this subscription also offers an unlimited 30-minute pick-up for or…