There are usually six small squares on the envelope. Just write the numbers in it, for example, 2765 18.

China postcode coding rules;

China adopts a four-level six-digit coding system, with the first two representing provinces, municipalities and autonomous regions, the third representing postal areas, the fourth representing counties and cities, the last two representing the post office, and the last two representing the delivery area of the city, that is, the location of the delivery area.

China's postal code is relatively simple, as long as it consists of six digits, there are only three kinds of verification error messages:

The string provided by 1. is empty or does not exist;

2. The provided string contains invalid non-numeric characters:

3. The provided string length is not 6.