US Post Office is located at 114 Oak Tree Ave, South Plainfield, NJ. This business specializes in Post Offices. US Post Office is open Mon, 10am-4:05pm; Tue, 10am-4:05pm; Wed, 10am-4:05pm; Thu, 10am-4:05pm; Fri, 10am-4:05pm; Sat, 8:05am-1pm; Sun, Closed and accepts American Express, Discover, Mastercard and Visa.