How is printed matter defined when the post office mails letters?

Printed matter is charged by weight, which is generally much cheaper than ordinary letters. When you arrive at the post office, people will tell you that if it is confirmed to be printed matter, the post office will stick a printed matter on the surface of the envelope for you. Generally, you don't stick a stamp, just stick a postage paid stamp on it. If the quantity is large, the post office will give you stamps and let you post them one by one. Hehe, I used to send hundreds of printed materials to the company a day, hehe,,,