Be specific, precise and concise by using professional key words used in business law.

Case:

X offered to deliver a parcel for Y to a downtown shop, as he would pass the shop on his way to work. Y agreed and gave X the parcel. The next day, Y told X he would give him $5 for delivering the parcel. X accepted Y's offer, but, later, Y refused to pay the $5.

What are X's rights?