Leti has just received a $15.00 gift card to Late, Late, Coffee for her birthday. Leti buys a small coffee each day. The total cost of the coffee is $1.50 including tax. The relationship between the coffee that Leti buys and the gift card can be modeled by a linear function y=15-1.50x. What is the initial value and what does it represent in the context of this problem?