Friends of ours arrived the day after we did, but through SXM. They rent directly from the owner, who, despite numerous requests, never provided a "letter". Neither Winair nor the gendarmes said anything. Could have been "policy" or could have been pure luck. For us, Tradewind were pretty by the book, so YMMV. As an aside, even though our friends had uploaded everything to the Sint Maarten website, the Dutch customs people apparently scrutinized their documents pretty closely. Happily there were no issues.