M-W:
frank
1 a : to mark (a piece of mail) with an official signature or sign indicating the right of the sender to free mailing
c : to affix to (mail) a stamp or a marking indicating the payment of postage
OALD:
frank
> verb [vn] [often passive] to stamp a mark on an envelope, etc. to show that the cost of posting it has been paid or does not need to be paid
CALD:
frank verb
to print a mark on a stamp so that the stamp cannot be used again, or to print a mark on an envelope to show that the cost of sending it has been paid
Chambers:
frank
verb (franked, franking) to mark (a letter), either cancelling the stamp or, in place of a stamp, to show that postage has been paid.
DWDS:
frankịẹren, frankierte, hat frankiert 〈ital.〉 eine Postsendung f. eine Postsendung freimachen, auf eine Postsendung die erforderlichen Postwertzeichen aufkleben:
einen Brief f.Wahrig:
* fran|kie|ren [ital.] tr. 3 mit Briefmarke(n) versehen oder mit der Frankiermaschine stempeln, freimachen
Dictionary: to frankDictionary: frankable