August 4 at 8:02 PM

A U.S. Postal Service letter carrier loads mail. (Luke Sharrett/Bloomberg)

The boom in Internet retailing has provided a rare bit of good news to the beleaguered U.S. Postal Service, which is delivering packages at a historic pace — and reaping revenue that helps offset billions of dollars of losses elsewhere in the agency’s budget.

But the Postal Service isn’t the only one making money when a mail carrier brings a new pair of shoes or a school backpack to your door.

Private companies compete fiercely to sell postage, prepare labels and help retailers find the best deals for shipping their products. And several of them argue that a single company,, is getting unusually good deals from the Postal Service, allowing it to undercut the prices of rivals and reap profits that should be going to the government.