Not sure if this applies, but "Products which are not suitable for return due to health protection or hygienic reasons if unsealed by you after delivery" are excluded from Amazon returns policy.

However - if you return the item to the customer, or simply do not refund, and even if you report the customer to Account Health for a violation of returns policy then any A-Z Claim put in by the customer will be automatically found in their favour, because A-Z don't look at anything other than the tracking on the return.

My advice would be to do a partial refund with a deduction for use and loss of value (as you really can't resell it as new). I had a conversation once with Seller Support (back when they could actually say things and not just send email templates with the name changed) about how 50% was the minimum they expect to be refunded - we have done that on occasion and it certainly stops A-Z Claims from auto-refunding and you can then argue the case to A-Z about the reduction in value.

If the customer contacts you about why they haven't been refunded in full, you may find the following paragraph of use:

"Please note that under UK law customers have a duty of care to ensure that goods remain complete and unused whilst they are inspected to determine suitability for use; customers are entitled to inspect an item in so far as they would be entitled to do in a shop, and have no right to open, use or damage an item and/or return it incomplete, damaged or bearing signs of use for a full refund of the order value, this includes manufacturer packaging. Please note that Amazon’s terms and conditions (as well as The Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013 Section 34 (9)) entitle the deduction of a sum appropriate to any reduction in value as a consequence of customer use or action."

Hope this helps :)