Amazon’s drone program proposal is to deliver packages to customers in 30 minutes or less. Now, Amazon is offering drone deliveries to Amazon Prime customers who live within 5.2 square miles around a small fulfillment center near Cambridge. At this moment it’s a private trial with two customers, during tests. Amazon debuted its drone program in the U.K. because it obtained regulatory approval more quickly. In order to qualify for drone delivery, the items must weigh less than five pounds. Last month a company started delivering Domino’s pizza via drone in a small test in New Zealand, while United Parcel Service Inc. in September kicked off a commercial drone delivery test to target deliveries like medicine for difficult-to-access or remote areas. However skepticism is strong. ”It’s an interesting logistical concept, but not particularly viable in an urban environment,” said Ivan Hofmann, a former FedEx executive.