This is something Shopify will do for you. Hoplix will not communicate directly with your customer. When an order is shipped, Hoplix will notify Shopify and Shopify will automatically send the mail to your customer.
In case you place manual orders that do not include your customers' email addresses, you can always find the shipped orders in your Account section, under Orders. Within the order details page, you will find a tracking URL (for all tracked orders) that you can manually share with your customers.