1. Postcard
葉書
Write the recipient's name, address, etc. on the front of the postcard. Also, write the sender's name and address at the bottom left of the front, or at the end of the back. Write the content (message) on the back. There is no problem with vertical or horizontal writing, just write in one style. Recipients find it easier to read if they are unified.
https://www.sourcenext.com/product/nenga/contents/manner/

2. Points to note
注意点
Write the recipient's name in large letters in the center. The recipient's address should be written from the right, but in smaller letters than the name. If you are sending to the recipient's company, write the company name/department name between the address and name.
https://www.shouken.co.jp/nenga-knowledge/nenga/business.html

3. Envelope
封筒
The basic writing method is the same as on the front of a postcard, but the sender's name and address can be written on the back of the envelope.
https://shimojima.jp/shop/pages/featurepage_a4fuutou_kakikata.aspx

4. Points to note
注意点
Sentences that would be considered rude by email should be sent as a sealed letter in an envelope. Examples include お礼状（thank you cards）, 招待状　（invitation cards）, 案内状（information cards）, and 詫び状（apology cards）. When doing so, it is important to write the name of the partner company formally, without abbreviating it. Rather than abbreviate it to （株）, write　株式会社. Please be sure to check whether 株式会社 should appear before or after the company name before writing it.
It would be very rude if you misspelled the name of the other party's company or the person in charge. Please be very careful not to make any mistakes in writing. If you make a mistake, be sure to rewrite it on a new envelope instead of using correction fluid to correct it.
Also, do not use pencils or mechanical pencils as they may erase your writing. Use instead a black ballpoint pen or felt-tip pen.
In addition, it is a good idea to write something like 請求所在中（Invoice included）on the bottom left of the front of the envelope so that receiver can see what's inside. It is a nice consideration for the other person.
https://found-er.com/column/job/5596/

Have you ever write postcard? Share in the comments!