Basic interview. How would you solve problems outside your expertise? How do you solve problems. What are your expectations. Nothing fancy. Be interested. Ask about the job. Show some knowledge of your employer. Lots of info online.
You hear the phrase "going postal" now and then. You know what it means. I'll tell you right now, postal workers are some of the friendliest, most helpful I've ever known. They know you're not taking their job. I have never had a person not give me good advice - solicited or not - when I needed it.
Now this is a union environment. These people know the rules, and if so much as a hangnail is out of place or someone of lesser seniority gets OT, you'll know it, and hear about it. Often. Not a deal killer. Good to know the rules. They protect the USPS and the employees.