If you have selected a tracking mail to a given shipping method, a button will appear on your order to Resend Tracking mail.
On your shipping method, select a given tracking mail from Webshipper.
The url should look like this:
Once it has been enabled on the shipping method you can open an order with that shipping method and see a resend tracking mail button.
When you click the button, no pop up will appear. The mail will just be sent instantly and a green notification bar will appear shortly to inform you that it has been sent.