yes you can mail them by taking to the post office. USPS is the carrier. i have done it and several others here can vouch for it. You are confused by the process. Your friends/family are not going to get access to Attorney General, US attorney, USCIS to be able to serve them. You are confusing civil case filing process vs immigration cases
This is why if you dont understand the legal process , go to a law firm and let them file the process