there are many examples in description:

solution(800); // should return '8:00'
solution(1000); // should return '10:00'
solution(1451); // should return '14:51'
solution(3351); // should return '33:51'
solution(10000); // should raise an exception

Why you sort gifts every time buyGift method is called? Without calling addGift method you always have sorted gift list on each buyGifts call except first time.