Is there a post-retirement benefit to accepting Federal employment (specifically the US Postal Service)?

The USPS has gone to FERS, no longer CSRS. You would not have to buy back any time as long as you don't use it to calculate your PS retirement. FERS is a SSA based retirement with a Thrift Savings plan add-on. Under FERS, an employee who meets one of the following age and service requirements is entitled to an immediate retirement benefit: age 62 with five years of service, 60 with 20, minimum retirement age (MRA) with 30 or MRA with 10 (but with reduced benefits).

You can retire from the Armed Forces, go to work for the USPS and gain sufficient time in service with the postal service for a second retirement. But you would have to buy back your Army time to have it combined as a Civil Service retirement--as I had done--20 years Army, 14 years in USPS for Civil Service retirement totaling 34 years.
But the "buy back" will be expensive. Mine was $10,000 in 1995. Your best choice is to see your personnel representative in the USPS for up-to-date answers.

Bear in mind you could retire from the USPS in normal circumstances, but I chose Civil Service because if I died before my wife, she could get part of my Civil Service annuity. Drawing from Army retirement for the widow might be minimal in amount.

Good luck in resolving your issue.

I actually purchased 10 years of CS retirement credit for $10K and it now pays me $13K a year more in my CS retirement. There's no connection to my Reserve retirement as it's under a different statue vs. AD retirement. So any Fed career is open. You just have to make sure under the current rules you aren't working against yourself which I believe you won't be.