Hi,
If you didn't send via trackable method, then there's a risk they open an Item Not Received dispute. Or open a SNAD dispute having found fault with the item.
If they were to do do so then they would need to return via on-line trackable method and at their own expense if outside the UK - which may not be cost-effective for them.
If within the UK, then under Distance Selling Regs the buyer is within their rights to cancel the order within 7 days of receipt. If you don't state in returns policy that buyer pays cost to return for change of mind, then you have to reimburse that cost too.

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