The shop is inside the gate so you must buy tickets
If you do go they are still using "timed" admission. If it's a slow day they'll probably let you in, or just go online and book it real time. Note: admission is free if you're a member of the Trustees of the Reservation. Check their website for more info.
You have to have tickets to get into the grounds at all, and the shop is out of sight of the gate by the parking lot, so I doubt it.
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