I am not sure I entirely understand your question. Why were there only $0.60 in pennies? Were pennies a smaller amount than a cent? Anyway, I have to say I think the main emphasis of this detail isn't on the breakdown of how Della had managed to save this amount. O. Henry is trying to underline their poverty by indicating how much Della and Jim are struggling financially. Note what it says in the first paragraph:
One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies. Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher until one's cheeks burned with the silent imputation of parsimony that such close dealing implied.
Note how the author reveals to us just how poor Jim and Della are. The short, sharp sentences that open the tale inform us how little Della has been able to save, even though she has tried her best and embarrassed herself by bargaining closely for their daily goods. Still, in spite of all of these efforts, she is left with one dollar and eighty-seven cents to buy a present for her husband, and not a penny more. It is this that O. Henry is trying to convey, rather than anything else.