Example of excluding a couple of checkout pages from Pantheon caching in a WordPress must-use plugin:

pantheon-wp-mu-cache-exception.php

<?php

if ( isset( $_ENV['PANTHEON_ENVIRONMENT'] ) ) :

	// Disable caching on checkout pages as per https://pantheon.io/docs/cache-control/
 	$regex_path_match = '#/foundation/donation-options/(checkout|shopping-cart)#';

	if (preg_match($regex_path_match, $_SERVER['REQUEST_URI'])) {
		// Use a later priority to make sure it runs after native Pantheon caching
		add_action( 'send_headers', 'cth_add_header_nocache', 50 ); 
	}
	function cth_add_header_nocache() {
		header( 'Cache-Control: no-cache, must-revalidate, max-age=0' );
	}

endif; # Ensuring that this is on Pantheon