Yes, for citizens and residents of the United States, traveling to St. Croix (which is part of the U.S. Virgin Islands) is considered domestic travel. Because the U.S. Virgin Islands (USVI) is an unincorporated territory of the United States, flights between the mainland U.S. and St. Croix are treated similarly to interstate flights. U.S. citizens do not require a passport to enter St. Croix, though they must present a valid government-issued photo ID (such as a Real ID-compliant driver's license) to board their flight. However, there is a unique caveat: the USVI is a "separate customs territory" from the mainland. This means that when you depart St. Croix to return to the U.S. mainland, you must clear U.S. Customs and Border Protection at the Henry E. Rohlsen Airport. During this process, you may be asked for proof of citizenship (a birth certificate or passport is recommended just to speed things up, even if not strictly required). Furthermore, there are specific duty-free allowances for goods brought back from the islands ($1,600 per person), which is higher than the standard international allowance, making it a "domestic" trip with some international-style perks.