You might want to notify your customers about fulfillments directly from Shopify instead of Webshipper.
Edit Shipping Confirmation Notification in Shopify
In Shopify, head over to Settings > Notifications and press 'Shipping confirmation'.
Edit your shipping confirmation template as wanted and press 'Save'.
Setup Shopify Notification in Webshipper
To activate your shipping confirmation emails from Shopify in Webshipper, navigate to Settings > Order channels and pick your order channel.
Press 'Edit order channel'.
Make sure that your mark the 'Notify customer' checkbox and press 'Update order channel'.
Your Shopify shop will now notify your customers when their orders are fulfilled.