Well, this can be gone back and forth on - personally, I think that you should write a thank you note, regardless. Someone went out of their way to find something for you that they thought you’d like…regardless of the cost, they were thinking of you.
On the other hand… if your neighbor dropped off a plate of cookies, I don’t know that it necessarily deserves a long drawn out note, but if it was out of the blue… tradition shows that it deserves a mention of some kind.
So all in all- when in doubt, write a note.
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