All baggage bound from Hawaii to the U. S. Mainland is subject to pre-flight inspection by the U. S. Department of Agriculture. Restrictions on the movement of fruits, plants, live snails, and other items from Hawaii to the Mainland are enforced to prevent the spread of fruit flies and other hazardous plant insects and diseases.

Plants may be purchased at certified nurseries, which seal and stamp packages containing the plants so the plants pass the inpsection at the airport. There are lots of certified orchid nurseries. Or you may have the plants mailed to your home for convenience. In addition, some stores, particularly those in tourist areas such as ABC Stores, carry plant material (such as small orchid plants, small ginger plants, ti leaf "logs", plumeria cuttings) packaged in plastic bags that have been certified and would pass the agricultural inspection at the airport.

How about the other way around? Is there any restriction on mail order plants TO Hawaii? Or seeds?

Mail order to Hawaii...

Call the Dept of Ag first and ask them if the plant you are considering is an invasive. If it is an allowed plant there should be as few leaves as possible and no soil. The sender must label it with the name of the plant and a label that states "may be opened for Ag inspection". This info was given to me by the Dept of Ag here on Maui.
Please do find out if it is considered invasive first!