This is something Shopify will do for you. SPOD will never communicate directly with your customer. When an order is shipped, SPOD informs Shopify about this and Shopify will automatically send the mail to your customer.
You can see your shipping confirmation email via Settings > Notifications > Under shipping, Shipping confirmation email.
In case you place manual orders that do not feature your customers' email addresses, you can always find the shipped orders in your SPOD App under Orders. Within the order detail page, you will find a tracking URL (for all tracked orders) that you can share manually with your customers.