My lovely friends often bring me treasures from Japan that are office-y or mail-related which is how this beautiful book came into my hands. The title of the book is New Postal Style 50 and is filled with envelope-like folds for mail.
All the pages feature variation alternatives to the same old envelope. I was smitten until I started looking more closely at the possible durability or mailability of these envelopes. I know that anything with a stiff stick cannot be run through the standard machinery used by the US Postal Service but the envelope + folded note design might be interesting for a letter.
There are some great ideas that might be modifiable for actual mail. I know that generally speaking, the postal service does not like string or rubber bands on the outside of a package either but maybe the wrap over a box could be modified with washi tapes and glue? The trapezoid letter is super cool, as is the stamped letter design.
In the back of the book are instructions and diagrams. Even if you don’t read Japanese, the diagrams and calculations are universally useful.
This book could be ordered from Amazon.co.jp or check with your local Asian book store so see if they stock it or could order it for you.