Mystery continues to surround the U.S. Postal Service's investigation of Amazon.com. For those who missed it, Amazon disclosed in its annual regulatory filing Friday that the postal service is "investigating our compliance with Postal Service rules, and we are cooperating."

Peter Rendina, a Washington, D.C.-based spokesman for the U.S. Postal Inspection Service, said in an interview Tuesday the investigation is "ongoing," but declined to provide details. Some, however, are pointing to the internet retailer's use of bulk mail or media mail services as a possible focus.Ron Wiener, CEO of Seattle startup Earth Class Mail, which works with the U.S. Postal Service, said he had no direct knowledge of the Amazon investigation, but had a few thoughts.

"The USPS is absolutely desperate for revenue so one of the very first things they'll do is go after major accounts and see if there's any revenue leakage," said Wiener, whose company scans postal mail so customers can read it online.

Wiener said it's plausible that the investigation has something to do with the postal service's manifest mailing system for bulk shipments.

"While it's hard to imagine that Amazon would do anything inaccurate, because they are accuracy freaks, it's possible that if they're doing fulfillment for smaller resellers, it's hard for Amazon to know if the weight calculations are correct," Wiener said.Some comments on TechFlash and postalnews.com, a site frequented by postal workers, have speculated that Amazon, or those who sell through Amazon, are using the Postal Service's Media Mail for items that don't qualify.

Media Mail is meant for mailing "books, sound recordings, recorded video tapes, printed music, and recorded computer-readable media (such as CDs, DVDs, and diskettes)," according to the postal service website.

Amazon said it learned of the postal service probe in January 2009. The company has declined to comment beyond the bare-bones statement in its regulatory filing.The U.S. Postal Service is lately grappling with mounting losses amid the growth of electronic communications and the worsening economy -- and recently proposed cutting back mail delivery from six days to five. Amazon is clearly a big business customer for the postal service -- so the stakes are high. We'll continue to follow this story as it develops.