WASHINGTON — Despite an increase in revenue for nearly all of the Postal Service’s products, the agency continues to be mired in debt because of a congressional requirement to pay about $5 billion into a future retiree health care fund, post office officials said on Monday.

In its latest quarterly filings for the period ending June 30, the post office reported a net loss of $2 billion, up from a net loss of $740 million over the same period last year.

But the service did report operating revenue of $16.5 billion, a $327 million increase over the same period in 2013. Post office officials attribute the growth to an increase in mail prices that started in January, new sales and marketing initiatives as well as continued growth in its shipping and packaging business.

“A lot of this growth is attributed to changes we’ve made such as price increases and the efficiencies that our employees have made over the last few years,” said Patrick R. Donahoe, the postmaster general.