After the loving couple have sacrificed their most prized possessions out of love for the other, proving that they understand the true meaning of Christmas, and proving, as O. Henry narrates, that they "[O]f all who give and receive gifts....are the wisest," Jim tells Stella that their presents should be saved until later as "They're too nice to use just at present. I sold the watch to get the money to buy your combs."
Of course, Stella's beautiful combs cannot be worn presently; for, she has sold her luxurious, long hair in order to procure the money with which she purchases the handsome watch fob. However, her hair will grow back to its length with the passage of time; so, Jim puts them away until they can be given to his wife for her use with longer hair than she has now.
In the end, we see that our characters are viewed by the narrator as "rich," not for their wealth or possessions, but for their spirit of generosity and love.