You're not going to like this heihachi, but I'm in complete agreement with Stalks, Dave and lyndal on this one, the intention of the concession stamps is for private mail for pensioners not for business use. 

You may bring up the pollies with their snouts in the trough with the benefits they rort on a regular basis, but really are you being any different, albeit you're only using 50 pensioner concession stamps a year.

Sorry, but any rort gets up my nose.