When a store receives an order email complaint like the one you shared, this means that a buyer from your store has visited the Contact Shopify Support page here and has selected the ‘Get support as a customer of a Shopify store’ option on that page. If the buyer chooses to submit a complaint about your store or their specific order, they’ll be directed to complete a complaint form. Once the form is completed, all of this information is then sent through to the merchant via email, and from here any correspondence to resolve the buyer’s complaint will occur between the buyer and the merchant.
You can find more information on how Shopify helps customers on the following resources: