Mailing a 1-ounce letter from anywhere in the United States to Hawaii costs 49 cents. Postal rates are uniform between all 50 states, so there is no additional cost to ship items to or from the mainland with USPS.
Currently, postal rates are 21 cents for every ounce over the first for letters and 34 cents for postcards. Parcels may vary in their cost of shipping due to the increased distance, but USPS Flat Rate Priority Shipping rates are the same to the non-continental states. Letters crossing the ocean may take slightly longer than normal to be delivered, however.