Sorry to hear you’ve had to deal with this difficult situation for your business.
Chargebacks are a tough part of selling online for merchants to have to deal with.
It sounds as though you have already been through the chargeback process in this instance, but for future reference you can learn about managing chargebacks here on our help site.
You would have been given the opportunity to present evidence in support of your case, to show that the product was sold as intended.
Ultimately, as you will have seen, these investigations are undertaken and decided by the customer’s bank or financial institute and are not something we have any control over from here in Shopify.