noun
postage deferred payment mail
A postal service where the sender does not affix stamps; postage is settled later, often used by businesses for bulk mailings.
料金後納郵便を利用すると、切手を貼る手間が省けます。
Using postage deferred payment mail saves the trouble of affixing stamps.
料金別納郵便 is a similar service where postage is paid separately but not necessarily deferred; 料金後納郵便 specifically means postage is paid after mailing.
Compound of 料金 (fee), 後納 (post-payment), and 郵便 (mail).