Postal codes are only available for the USA and Canada. About 85% of the IP addresses allocated to the USA have postal codes in our database. Likewise, about 70% of the IP addresses allocated to Canada have postal codes in our database. In most cases we are able to identify the city, but cities often have multiple postal codes. We don't identify the postal code in such cases unless we are confident that the IP address is likely to be in that postal code and not one of the others in the city.
Not being able to determine the postal code in many cases is a general limitation of geolocation services. Typically an ISP will have its users connect to a central office and assign their client an IP address from a block of IP addresses. The block of IP addresses will be given out to clients spanning several postal codes. For instance, if you connect to a dial-up ISP in New York City, your provider will typically have a 212 access number that is used from any postal code in the city.
With a postal code database you could lookup a US postal code using the city and state returned by our service. Likewise, you could use the city name and region code to lookup a postal code in any other country. You can easily find postal codes databases on the internet.