SmartLetter is an envelope sold by Japan Post. The envelope is relatively hard and it is not so easy to be teared or folded. Then, it can be send without putting a stamp on it due to the envelope includes postal fee. It is sold JPY 180 per envelope.
The size is 250 mm x 170 mm and maximum width is 2 cm. Then, the maximum weight is 1 kg. Therefore you can send CD or DVD to your friend without worrying the postage and also buying an envelope. However, it is only for domestic mail, not valid for air mail.
Thanks to fixed postage, you do not have to bring it to the post office or weigh the package and buy the stamps for the weight and size. Then, you do not have to go to the post office during its open hours. You can put it into the post box anytime you want. Then, postman delivers SmartLetter to the letter box.
Japan Post also sells LetterPack which is similar to SmartLetter but the differences are the size, LetterPack is bigger. It is 340 mm x 248 mm, which is suitable for A4 size document and the maximum weight is 4 kg.