That is NOT the official government website and you DO NOT pay for the postal exam
Read the USPS website http://about.usps.com/publications/pub60a/welcome.htm#508hdr5
"Test dates for test 473 are publicized widely, including as follows:
• On the Postal Service Internet home page: www.usps.com/employment.
• On public bulletin boards in Post OfficesTM and in local, federal, and state municipal buildings.
• At State employment offices.
• On local television, in the newspaper, and in radio advertisements.
• Through community groups, including minority, women, and veterans' organizations.
How to schedule yourself to take test 473 . . .
During the opening period indicated on the test announcements for test 473, schedule yourself to take test 473 by accessing the job posting on www.usps.com/employment or by calling the toll-free telephone number provided. The Internet and phone system prompt you through the application process, request the announcement number, and ask for other required application information.
At least 1 week before the test date, we will mail you a scheduling package that indicates when and where to report for the test. We will also include applicant instructions and sample questions."