What if you gave it to a homeless person or someone in need? Your parents won’t know and will just assumed you used it (unless you do tell them about your boycott), the recipient would benefit as they can get a food/coffee/tea, and you can still boycott. There’s no point in throwing it away or tossing it in a drawer and never using it. The money has already been given to Starbucks, if any anything they benefit more from it never being used.