Hey there, your not the first to ask about this.
In what form is the data? If it is in xml or JSON format or has an option for that you can.
You have BC system tags to return the persons IP and also country code to be used in this regard as well.
The in's and out's of course are all dependant on the exact needs and what you are doing.
XML and JSON are just data formats, they are neither slow or fast.
Here is a high level overview of what needs to happen in order to do what you are asking:
-The web service will query the database for the information you want (in your case the postal code that coresponds to the lat/long you provide).
Also, since the webservice can't be hosted on your site (BC doesn't have any way to do that), it will have to be on another domain and hosted seperately from your primary site. Something like webservice.example.com instead of www.example.com. However because of this, the browsers built-in security system will take over, and not allow the data from the external service to be consumed. To get around this limitation you will need to configure your webserivce to send data as JSONP, or use CORS, which is unfortunately not supported in some older browsers.
There are lots of differently ways to create and host a webservice.
However, I'd first look for a pre-built service that does what you want.