There is no need to re-engineer host applications if you want to use barcodes. The EscapeE barcode plug-in will work with any PCL/HPGL print file. Just mark-up the fields containing the barcode data and EscapeE will rewrite the document complete with the barcode ready for printing.
The British Post Office 4 state code is used to encode the Post Code (The UK equivalent of a ZIP code). It is used by automated sorting equipment. It has some limited acceptance in other European countries. The U.S. PostNet uses a pattern of 4 short or tall (2 states) bars to encode digits. BPO uses four states - tall, short or medium bars extending upwards and a medium size bar extending downwards.
The BPO 4 State code has a fixed height and width.