The U.S. Postal Service announced Thursday that it will offer $20,000 early retirement incentives to eligible administrative staff as part of a money-saving strategy that also will close seven district offices from New England to New Mexico.

The cuts are part of a reorganization to eliminate 7,500 administrative, supervisory and postmaster positions this year to help the Postal Service address record losses from declining mail volume. Once buyout decisions are final at the end of April, the agency plans to eliminate the jobs of thousands of postmasters and supervisors, many through layoffs, officials said.

“Nobody did anything wrong, but we’re a victim of the economy and past legislation,” said Anthony Vegliante, the Postal Service’s chief human resources officer and executive vice president. The cuts are expected to save $750 million in the next year.

Also Thursday, a long-awaited advisory opinion on the Postal Service’s proposal to do away with Saturday delivery suggested that the agency underestimated the effect on service of five-day delivery and overestimated the cost savings.

Among the findings of the 211-page opinion from the Postal Regulatory Commission:

-- Five-day service would delay by two days delivery of 25 percent of First Class and Priority Mail.

-- The Postal Service did not adequately evaluate the effect of five-day service on rural areas.

-- While the Postal Service estimated net savings from the reduced service at $3.1 billion, the estimate should be closer to $1.7 billion.

-- Lost revenue from volume declines from the service cuts would be $600 million a year, not the $200 million the Postal Services estimates.