SAN FRANCISCO - A year ago, Amazon.com began quietly opening a few delivery lockers at convenience stores, supermarkets and drugstores in Seattle, New York, and just outside of Washington, D.C. This year, the online retailer will install its 50th Amazon Locker in the San Francisco area, the Wall Street Journal reports.

The metal cabinets address concerns apartment dwellers in particular have about ordering packages for home delivery that arrive while the occupant is not at home. "The home-delivery challenge has always been an issue for e-commerce in Europe and Japan, and is growing in the U.S., especially as thieves have moved into the game," said Fiona Dias, chief strategy officer for ShopRunner. "It€™s easy to follow a UPS truck around and steal packages from doorsteps."

The delivery lockers work much the same as Walmart and Best Buy€™s online orders/in-store pick up services. But since Amazon has no brick-and-mortar locations, the company has been renting space from retailers such as 7-Eleven for locker space.

Customers whose orders are shipped to a locker delivery site receive an emailed code that allows them to unlock the door to get their merchandise. "When customers ship Amazon orders to an Amazon Locker, they can pick up their packages at a time and place that's convenient for them," said an Amazon spokeswoman.

The four U.S. cities that host Amazon Lockers are in states that already collect sales taxes from Amazon customers, or will soon begin charging sales tax. Amazon is staying mum on where its lockers will appear next. Currently, Amazon also has lockers at locations in the United Kingdom, and is "adding new Amazon Locker locations every week," its website reports.