u s postal service tracking number
USPS Tracking Number Format. The most common USPS tracking format is a 20 digit number (e.g. 9999 9999 9999 9999 9999), or a combination of 13 alphabets and numeric characters. Which is usually starts with two alphabets, and it is followed by a 9 digit number and ending by the US (e.g. EA 999 999 999 US).