To force Standard Post on all USPS shipping where no other shipping method will be available for Domestic shipping, you need to edit the USPS shipping module:
/includes/modules/shipping/usps.php

and change the code in RED around line 806 to true:
// force GroundOnly results in Stand Post only being offered
// to use, you must have a field for products_groundonly in the products table
// then uncomment the in_cart_check for the products_groundonly
// when $usps_groundonly is set to true, only Stand Post will show
$usps_groundonly = 'true';
// $usps_groundonly = ($_SESSION['cart']->in_cart_check('products_groundonly','1') ? 'true' : 'false');
Now, regardless of what shipping methods you have checked, be sure Standard Post is checked, only Standard Post will work for all USPS shipping and no other shipping method will work ...