It's not what you heard, the detention center can communicate, and the detainees can buy envelopes, writing paper, pens, stamps and other things from the officers at their own expense (sold on the same day every week). It's just that the content of your communication doesn't involve the case. Both registered mail and express mail can be received.

　　In addition, clothes and money can also be sent to detainees. There can be no metal decorations on clothes and no straps on shoes, in order to protect the health of detainees (for fear that someone will commit suicide or use it in a fight). Money can be paid directly to the detention center in cash or sent by mail. After the money arrives there, there will be a special passbook to put their money. It is very safe to deduct the daily necessities (including envelopes and stamps) or buy food and other things and then sign it for confirmation.