Mike, the seasonal job are not permanent so there’s no civil service assessment. These are the requirements for those positions - (18 years old at the time of appointment or 16 years old with a high school diploma, United States citizen, permanent resident, or citizen of American Samoa or other U.S. territory, Recent employment history, Ability to pass a criminal background check, drug screening, and medical assessment, Safe driving record (if applicable to the position), Must be registered with Selective Service if applicable, see USPS Careers

I don't believe so Mike. Here's their application site online - note the "employee requirements" section. No mention of civil service exam.
Suggest you just stop into your local branch during off-hours and ask them though (I'd say call, but I just tried of you and its tough to get the phone answered). https://about.usps.com/careers/search-apply/welcome.htm