The general rule is that if you're a US resident and gone from the US for more than 48 hours, you can bring back goods acquired abroad up to $800 per person, once every 30 days. without having to pay import duty. Whether tax was paid abroad, or it was purchased "duty free", has no bearing on this limit. There are additional limits as to quantity for alcohol and tobacco.
As others have said, there is no sales tax for goods purchased on board the ship, as the stores are only open when the ship is in international waters. I'm not sure about drinks or spa services purchased when the ship is in port.