More often than not, these types of purchases require a credit/debit card. Check the merchant's website to see if they offer cancellation via email. If they do, instruct the customer to send one and keep a copy. If the merchant refuses to process, you can use the email as proof of cancellation and you will have chargeback rights if the merchant does not issue a credit within 30 days of the date on the email.
The cardholder is not entitled to refund of the shipping and handling unless the terms and conditions specifically state that they will be refunded as well.
If no email provision exists, I would recommend cancelling the card before the free trial expires and your customer is charged for product they do not wish to receive.
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