I set this up in a PCL context for someone a couple of years ago. As I recall, the fonts supplied by the USPS were simple bitmap fonts and only a few hundred bytes long as they only contain 4 characters and are not scalable. If you print out a typeface list once the font downloaded to the printer, you should get an appropriate command string.
That is all very trivial compared to the process to generate the data to print. It's not just a matter of loading the fonts and printing the zipcode.