How are Della and Jim the wisest "of all that give and receive presents"?

Jim and Della are very poor, and they live in a run-down, shabby apartment. But their marriage is incredibly strong, based as it is on a deep mutual love. Christmas is coming, and so Jim and Della decide to buy each other a gift. But since they're both so poor they will need to make sacrifices. On Christmas Eve, Della sells some of her beautiful hair to a hairdresser by the name of Madame Sofronie. With the money she earns, she buys Jim a platinum chain to go with his pocket watch.
But when Jim returns home, he notices that Della's hair is short all of a sudden. She has no choice but to own up and tell him about selling some of her hair to buy him a gift. Jim also has a confession to make: he sold his pocket watch to buy Della a new set of combs, now completely useless just like the watch chain that Della bought.
Jim and Della, though they both make sacrifices to pay for what turn out to be useless gifts, are still wise because they have realized that the love they have displayed is a much more precious gift than anything that money can buy.