Guests aren't allowed but you can book and pay directly. Payment is taken at the centre. I tried to do and got a booking, but it didn't happen as everything was disrupted the day of our flight due to the hurricane. But they drove us back to the main terminal. The experience felt shifty especially in comparison to a previous trip using the IAM centre in Grenada. But TBH that was the day after a sever hurricane where 25% of the island still had no electricity so I wasn't about to argue, I was merely glad our travel plans were not disrupted.