Tiny office in a row of tiny rental car offices on the right when you exit the airport. Booked based on reviews.
Great experience, super friendly staff at check-in (Josh and Val). It took longer than normal since we had not prepaid and I had to get a Local Driver Permit (22 USD).
They do not honor Expedia purchased CDW. I highly recommend buying Sixt's Top cover CDW, CDW, and Tire/Windscreen insurance (we did not have any damages, but there is a higher than normal chance you will, due to the road conditions).
Make sure to rent a more powerful car (like Suzuki Grand Vitara Hybrid) otherwise it will struggle to get up the steep and very steep inclines. You need to be a very confident driver since you drive on the left side with steering wheel on the right (like England), and many of the roads are treacherous, have hairpin turns, and single car width at times. Many of the top locations are at the end of these exceptionally bad roads, and you will wonder if you are on the right path. Google Maps only failed us once when it tried to guide us the wrong way down a one-way road in Soufriere. Road markings are infrequent.
Their cars are parked right across their office in the general parking lot. This is the same place you return your vehicle, and that was a smooth short process, again very friendly lady, she will inspect your car for damages.