The biggest issue is that you'll have to get (aka purchase) the PostNet barcode font, and install in on the appropriate computers. sounds like the web server, possibly the end-users workstations. And, of course, your development computer.
The second thing is that the data will have to be in the proper format. If memory serves me, it is Zip5+4+2 with periods surrounding this data.
Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes