Following the rather bittersweet moment at the end of the story when Jim and Della both realise what they have sacrificed for each other, the final paragraph of the story is written in a kind of rejoicing, optimistic and heartfelt tone that points towards the way in which the sacrifices that Jim and Della both made, although they could be judged as foolish and indicative of stupidity, actually are the closest to the origins of giving Christmas presents, because of the sincere love that such actions represented. Let us note how this passage ends:
But in a last word to the wise of these days, let it be said that of all who give gifts, these two were the wisest. Of all who give an dreceive gifts, such as they are the wisest. Everywhere they are the wisest. They are the Magi.
The rejoicing tone is evident in the way that Jim and Della have defied the wisdom of the age, and by so doing become "the wisest" themselves, as they capture the meaning of Christmas and represent a harsh commentary on materialism and what it has done to this important festival, which at its heart is all about love and sacrifice.
The final passage carries a central, overall knowing or prophetic tone, but there is a hint of an almost satiric or disingenuous tone within. This quote in the last passage is offered by O.Henry to punctuate the knowing tone by adding a bit of condescension toward the reader. It serves as a silly contrast
"And here I have lamely related to you the uneventful chronicle of two foolish children in a flat who most unwisely sacrificed for each other the greatest treasures of their house."
The reader can almost hear the narrator smiling in this last passage. The narrator is enjoying the nature of the beauty within the unselfish sacrifice made by each the man and the woman. In the end the narrator embraces and rejoices in the recognition of these two people as the magi, thus recognizing his feelings of the TRUE nature of gift giving, of love: disregard of self and focus on other.