It's at the airport but you have to take the shuttle bus to where you get the rental. It was convenient for us and all rental car places are in the same area.
The shuttle bus to the car rental area runs continuously at Logan. You do not need to reserve or call anyone. Just look for signs to car rental to find where you wait for the shuttle bus. The bus stops outside every baggage claim area at every terminal. Wait time could be 1 min to 20 but I do not remember waiting more than 10 minutes ever. Time to the car rental area depends on which terminal you land at, roughly 5 to 10 minutes from pick up. It is common to tip the uniformed workers who help lift heavy suitcases onto the bus, especially in rough weather.
It would be considered offsite.
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