I think that your switch statement is ok for displaying the service type. When it comes to getting the new flat rate shipping charges working the first thing you will need to do is add in the set session value server behavior. You can get to this in the eCart section of the server behaviors. When you add in the new session variable you can specify the trigger as the posted form element for the service select list you have. For the name just give it something meaningful like flat_shipping. For the value you can just set it to the posted select list value.
Once that has been written to the page you will need to customize it. For the condition you will need to update it so that it is not only looking for the posted element being set but also the two values that indicate flat rate shipping. So you will add in an and condition then some parenthesis to check if the value is equal to one of the flat rate values. It will look something like this when you are done with it:
if(($_POST['your shipping select list'] != "") && (($_POST['your shipping select list'] == '50') || ($_POST['your shipping select list'] == '100')) ) { // I used 50 for US and 100 for internationsl
$_SESSION["flat_shipping"] = $_POST['your shipping select list'];
}
Once you have got this session variable worked out you can then add in the new shipping rules. For the US just look for the presence of a session variable and specify the flat_shipping variable then also that the value of this session variable is set to '50'. You will do the same thing for the international but you will check to see if it is set to '100'.
Finally to edit the UPS you will just add in two more conditions, you will check to see if your flat_shipping variable is not set to '50' or not set to '100'. If these conditions are met then you should just apply the shipping quote that is returned.
If you have any questions about any part of this please let me know and I will do what I can to help clarify anything for you.