Buyer is a scammer
He ordered the item: "simplehuman 406 x 78 x 230 mm 20cm wall mount sensor mirror 600 lux ST3002", with Manufacturer's part number: ST3002.
He received the item on April 17th by DHL, tracking here:
https://www.dhl.com/it-en/home/tracking/tracking-express.html?submit=1&tracking-id=1381252681

He opened a return request by claiming that "Ordered modem ST3002 receive model ST3003" and he posted some photos showing the serial number of the product, that is L70239379.
At a firtst instance, in order to close the claim friendly and quickly, I offered the buyer a partial refund of 50 EUR, as a goodwill gesture and trusting his words.
Then I made a double-check on the item we had shipped to the buyer on April 5th, and I found out that the item we shipped was the right item, with Manufacturer's part number ST3002 and with serial number "L74821105" which is different from the one posted by the buyer.
The serial number of the product posted by the buyer also shows clearly that that item the buyer is pretending to have received from me, is a serial number of a much older batch of production, since it begins with "L70...", while the real serial number of the product I sent begins with "L74..."