file2.php will do:
1. Add Login = "My-Username"
2. Add Passwd = "My-password"
3. With Cust and Total from file1.html, file2.php post all the 4 value (Plus Login and Password) to destination-otherdomain.com (http://destination-otherdomain.com/receiver)
If you have to use the POST methode then you have to submit a form or if you want to use get method then a simple header() will do.
Now I guess this is for the post method.
You have to take all the values from the file1.html to hidden values in the file file2.php.
eg: <input type=hidden name=Cust value <?=$Cust?>>
and also you have to make the login and password files also as hidden filed then on a javascrip function you have to submit the form something like this
//all the php functio checking and hidden values ...........
//check of any condition by whic you can make sure the this file is called by file1.html.. probable by checking the submit button's value..
This will submit you form which stors all the hiiden values from the page1 and this page.
If this doesnot help get back to me..