同一差出人から同一受取人に宛てて郵袋という、文字通り袋に印刷物を入れて郵送します。

A mailbag from a single sender addressed to one recipient, as the name suggests, is for delivering a bag of printed matter.

印刷物には、特別郵袋印刷物というのがあります。

Included in the printed matter category is what is called 'special mailbag printed matter'.