President Trump tweeted in December that the U.S. Postal Service isn’t charging Amazon enough to deliver merchandise. But it’s not Jeff Bezos who is getting the lowest rates. In fact, it’s a major economic competitor of the United States—China—that is getting a sweetheart deal from the USPS.

I learned of this because it undercuts my company, Mighty Mug. We sell a patented travel mug that we mail customers from our warehouse in New Jersey. Over the past two years, China-based competitors have flooded the market with knockoffs...