Every time you speak to Shelly and request a rucksack upgrade, Shelly gains heart points - even if you don't have the items to create the upgrade. To easily get Shelly to 10 hearts, request the rucksack upgrade over and over when you don't have the necessary materials with you. This is useful because the amount of money you make working part-time at the Tailor Shop is based solely on Shelly's heart level; therefore, if you raise her heart level this way, you can easily make a lot more money in your part-time Tailor Shop work.