This is a good question. As you correctly point out, the two greatest examples of love are when Della cut her hair and sold it to buy Jim a chain for his watch, and when Jim sold his watch to buy Della combs for her hair.
Apart from these two examples, there are other examples of love. The first lines of the short story also speak of great love, because it shows that Della saved all her money all year to buy Jim a beautiful present for Christmas. In other words, her love was something consistent throughout the year.
"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 and parsimony that such close dealing implied. Three times Della counted it."
In terms of Jim, it says that that he was never late from work. The implication is that he worked hard his whole life and he was dedicated to Della. This shows great love as it shows faithfulness.