Regardless of Eastern, Western, 3-night, or 4-night, you only have to deal with U.S. Customs on the morning of debarkation. You fill out a form. If you exceeded the U.S. Customs allowances, you must pay duty on the excess (which you do on the ship, early in the morning). It's also possible for U.S. Customs to spot-check your checked or hand-carried luggage.
On the Eastern itinerary -- and only on the Eastern itinerary -- there's a U.S. Immigration check early in the morning. This is not the same as U.S. Customs. St. Thomas is part of the United States, and you're returning from St. Maarten, which is a foreign port. There is no similar mid-cruise U.S. Immigration check needed on the other itineraries.