As he hides in the bushes under Roxanne's balcony as she believes she is talking to Christian in Act III, this description and definition of a kiss by the superlatively poetic Cyrano de Bergerac employs much figurative language, among which is metaphor.  Cyrano also directs the action of a kiss to other parts of a person rather than the actual physical location. So, with these techniques in mind, the comparison of a kiss can be likened to other things not commonly thought of, but which are, indeed, relative.

For instance, rather than stating that the kiss is a promissory note or bond, perhaps the kiss can be the end of things anticipated. In this case, then, the kiss can be compared to other concepts and objects. One metaphor is that of the kiss being a seal of the union of hearts, the opening of the path of loving bliss, the key that opens the bud of love's future, etc.