I don’t want to derail this thread.
But “getting it in writing” is meaningless if, in fact, the OP’s grandparents need the money for their own health care. A written promise “we’ve made a commitment to fund our grandchild’s education” does not override any bills incurred for hospital care, and does not supersede whatever provisions have been made in a will.
Getting it in writing is a psychologically comforting idea which does not hold weight (fortunately, if you happen to be a creditor trying to collect from someone who insists they can’t pay because “I promised to pay for XYZ two years from now”).
OP-- you are very lucky to have grandparents on your side!!! Just make sure they understand what they are signing on for… costs have risen a lot since your parents were in college…