Another Letter

The official reply (click for a larger version)
In the previous post, I forgot one other letter I sent last week – a note to the post office headquarters. I got a reply this morning – a speedy three day turnaround. Hooray for Japan Post!
I had asked them why some post offices required me to fill out customs forms when sending books overseas, and others just stamped them with Printed Matter. I wanted to know which was the correct way.
It seems both are correct. Printed Matter can weigh up to 5kg, except for Canada and Ireland; Small Packet (which requires a customs form) is up to 2kg and valid everywhere except Afghanistan. The letter suggests I use whichever post office interprets this the way I want. So flexibly Japanese!