I believe the official tax rate on rental cars is 10%. It is definitely NOT 5%. I don't think SXM actually has a VAT. Regardless, don't stress over tax rates. Just get a quote from whoever you want, and ask them for a TOTAL quote, including all taxes and whatever insurance you want. Companies charge all kinds of different rates for different things. Don't sweat it. Get a total quote and make your decision.
I should point out that the quote with the 5% tax also has a 13% surcharge. I don't think it is necessarily gouging, but certainly obfuscating the price to get people who aren't paying attention to the bottom line. I booked a hotel in NY and there is a mandatory $10/night facility fee. Why not just bump the room cost up by $10 and call it a day? Because that makes their room price more than the surrounding hotels.
The difference between "marketing strategies" and "sales deceptions" can often get blurred in the SXM rental car business. Get written TOTAL quote and be prepared to produce same when you turn the car in at the end of your vacation. This approach should allow you to make an informed decision and eliminate unwanted surprises.