None of my international large letters contain printed material, either. 

I have always lodged my articles at the counter, customs forms filled in, international large letter - merchandise, as I explained above, with the description clearly explaining the contents were not printed matter.

There have always been some services, both domestic and international, where documents were the only allowed contents, but regular large letters were always excluded from that (small letters are a different thing entirely, and registered international large letters can only contain documents); but even documents has a broader meaning by Australia Post terms (they define a document as something that contains reproducable media, so it includes CDs, memory cards and USB sticks). 

As I said, today is the first day I have ever seen the very specific "written correspondence only" in relation to international large letter articles.