The Postal Service has raised mail prices five times since December 2020, when its regulator granted new pricing flexibility — and may soon be asking for a sixth increase that would go into effect this summer.

But a new industry report warns USPS is underestimating how much these rate hikes on its monopoly mail — or “market-dominant” — products are driving away some of its biggest customers.

A report from Keep US Posted — a nonprofit group of consumers, nonprofits, newspapers, greeting card publishers, magazines, catalogs and small businesses — finds market-dominant revenue in fiscal 2023 fell $1.8 billion short of the Postal Service’s own forecast, “signaling a potential issue with the model used to defend rate increases.”