Or send an hourglass, fine quicksand, and describe in detail the ups and downs they have gone through together as a witness of love.
In fact, I think the key is whether you have intention or not! Actually, the gift was not intentional. Gifts from the heart are the best gifts. Your grandparents will be very happy to know that you have this filial piety ~ they also know that their grandson is still at school and he has no money. Having a heart is the greatest comfort to the old man, knowing that he has never hurt you for so many years.