You have to say Heart Backpack in order to buy it from Valentina. She only has one left and as such any one character can only buy one from her. It was intended to be available on Valentine's Day, but the implementation failed and, being a weekend, CipSoft were not around to manually implement it. It first became available on the 16th of February. People can buy these backpacks for a lot of money, because you can only buy one per person.
See Backpack for general information regarding backpacks.