InPhonic is rapidly becoming the dominant retailer of online mobile service in the U.S., announcing this week it has won the Web portal mobile business of yet another big box retailer, Staples.

Staples joins Radio Shack and Sam’s Club on InPhonic’s customer list as well as online online retailers Amazon.com, MSN Mobile and AOL Mobile. While all of those brands run their sales sites under their own URLs, InPhonic is the white label portal powering the transactions and customer care. InPhonic offers similar services to some wireless carriers themselves, running off site sales and promotions, though the majority of operators maintain in-house the majority of their online operations.

“Our ability to add industry-leading clients, like that of Staples, reflects our reputation and strength in the marketplace,” InPhonic’s president of e-commerce Andy Zeinfeld said in a statement.

Though InPhonic was started in part as mobile virtual online enabler to help virtual operators launch service and handle their back office operations, its resale business has been growing at a much faster clip, at least on public announcements. InPhonic runs its own resale portal called Wirefly, which allows customers to compare price, features and performance of different carrier’s services all the way down to a specific ZIP code.