Remove the redirect page specified in the "store cart details in database". I think that is causing the page to redirect before the email is sent.
You will have to update the shipping email template to customize for USPS. UPS required you to enter the tracking number on their site, I haven't looked into USPS system, so it may be different. The email body is contained in an email template in the universal email directory. You can edit it with DW to change the appearance and functionality.