Amazon.com has quietly rolled out a new service to let customers return unwanted merchandise using large metal lockers it has installed for deliveries in garages, convenience and grocery stores in major metropolitan areas. The service will help address a problem that has plagued Amazon and other e-commerce retailers. As much as a third of all online purchases are eventually returned, by some estimates, making it costly for merchants that in some cases pay for shipping in both directions. Packaging and shipping orders is a major expense for Amazon. The company has been on a warehouse building frenzy in recent years, constructing facilities close to urban centers to speed delivery times. Amazon spent $8.59 billion on order fulfillment in 2013, up from $6.42 billion a year earlier. Amazon has been installing the lockers in public places over the past several years, giving urban dwellers a way to ensure their packages can be delivered even when they are at work.