After Della shows Jim the watch chain she's bought him and asks for his watch, he responds with a smile and an ironic comment:
"Dell [...] let's put our Christmas presents away and keep 'em a while. They're too nice to use just at present. I sold the watch to get the money to buy your combs."
When a character uses verbal irony, they say the opposite of what they really mean. In this case, Jim doesn't really mean that the gifts—the watch chain and the combs he's bought Della—are "too nice to use." Rather, the gifts are useless: Jim can't use the watch chain because he sold his watch in order to buy Della hair combs, and Della can't use the hair combs because she cut and sold her hair in order to buy the watch chain.
Jim's lighthearted remark is an understatement—a subcategory of verbal irony in which the speaker downplays a situation in order to highlight its magnitude. Della knows perfectly well that Jim doesn't mean these words literally; that is, there won't come a point when the gifts won't be "too nice" for everyday use. Rather, Jim is downplaying the harsh truth that they've both paid dearly for gifts that, because of that very generosity, neither of them can use at all. (Technically, Della could use the hair combs someday, once her hair grows out again. But given the combs' associations, it's a safe bet that she won't want to use them; it's at least as unlikely as the possibility that Jim will be able to afford another gold watch someday.)
This instance of verbal irony also reinforces Jim's mild-mannered character. Instead of reacting to the wasted gifts with anger or despair, he responds with mild humor. This doesn't mean he doesn't find the situation upsetting at all, but that he seems to be capable of putting things in perspective, and of trying to comfort the more emotional Della by defusing the tension.