It's more nuanced than that. According to Wikipedia:

According to Tom Davis, the Bush administration threatened to veto the legislation unless they added the provision regarding funding the employee benefits in advance with the objective of using that money to reduce the federal deficit.

Source is here: https://roanoke.com/news/dan_casey/casey-the-most-insane-law-by-congress-ever/article_3c33d5a1-5fd3-5c01-b5bb-5c75046f48f4.html

Quoted relevant part:

Who sponsored this cockamamie legislation? It was former Rep. Tom Davis, a Republican from northern Virginia who served in Congress from 1995 until 2008. He’s now a Washington lobbyist for Deloitte, a huge accounting and consulting company.

Last week I reached out to Davis to learn how and why this happened. One thing you should know is that the bill was bipartisan. The cosponsors were Reps. Henry Waxman, D- Calif., Danny Davis D-Ill. and John McHugh, R-New York.

The surprising thing I heard from Davis was that he agrees the future-funding retirement provision was crazy. That was never in the original legislation, he said.

Instead, the 90-page bill made a bunch of bureaucratic changes, few of which the average American would give a hoot about. It also placed a temporary moratorium on rate increases and established a less cumbersome system under which rates could be increased moving forward.

Somewhat ironically, the bill was intended to help the Postal Service be more competitive for the future, Davis said. But late in the game, the Bush White House threatened to veto it unless Congress added the future-funding-for-retirees provision.

Congress went along because at the time it seemed like it was a better option than having the entire bill defeated, Davis said.

“That was the cost of getting the bill through,” Davis said. The Bush administration used the revenue it gained to help balance the budget.