The only "preference" anyone gets for a USPS job is "veterans' preference" and there are several categories. 10 pointers can pretty well cruise right in provided they meet minimum requirements, and 5 pointers (like me) are right behind them.
I once hired over 100,000 returning Viet Nam War vetrans ~ many of them straight out of the boonies ~ gave 'em the test right in the field dodging bullets and mortars.
Well, enough on that. USPS has a pretty open hiring system. You take the test. You wait. The test is no big thing ~ just to demonstrate you can read and aren't going to walk out there in a modern industrial environment and lose limbs on the first day.