Many consumers are irked by the catalogs, credit-card pitches and other "junk mail" they receive. But the U.S. Postal Service loves it—and wants to deliver more.

The agency, beset by historic losses and a plummet in first-class mail, is running promotions, easing rules and planning television and radio ads to encourage more businesses to send pitches by standard mail, the official term for bulk mailings used by marketers to prospect for customers.

"What...