The results also reflect continued erosion in the delivery of first-class mail as well as expensive mandates for pre-funding of its retiree health care obligations.
The post office's report shows earnings of more than $1.4bn between October and December 2016. But when effects of a $1.7bn change in workers' compensation liability due to fluctuating interest rates are excluded, the service says it lost money overall.
Operating income came to $522m, down from $1.3bn in the previous year.
The post office continued to notch double-digit growth in its package business, boosted by the strength of Amazon and other Internet retailers, but also suffered losses from a forced reduction in stamp prices last year. Election-related mail volume, meanwhile, rose to 1.3 billion, up from 214 million in the same period in 2015, a nonelection year.
