[WooCommerce UPS Shipping] This script allows you to specify products as "already packed". They will not be added to separate boxes, and will not be combined with other products.

ups-self-packed-products.php

<?php
// This script allows you to specify products as "already packed". They will not be added to separate boxes, and will not be combined with other products.
// This is useful if you have products that are already packed and ready to ship that should not be put into another box.
// 
// IMPORTANT:
// The plugin 'WooCommerce UPS Shipping' does not currently support the filter "woocommerce_shipping_ups_skip_packing".
// For now, you must manually edit the UPS plugin directly. The two edits are located below (for version 3.2.18):
// 1. https://gist.github.com/RadGH/ad8d8e599366a27c81bc84dd1f66aaa9
// 2. https://gist.github.com/RadGH/b7b69f65bbeb7212083058d1d644b0c9
// 
// The rest of this script goes in your functions.php or similar location.

/**
 * When the WooCommerce UPS Shipping Method is packing cart items into boxes, examine each eligible product and see if "Packed Separately" is enabled.
 * When "Packed Separately" is enabled, the product's dimensions are used as a custom package size and the product is not combined in another package.
 * If multiple quantity is specified, more than one package will be added.
 *
 * @param bool            $skip
 * @param string          $cart_item_key
 * @param array           $cart_item_data
 * @param string          $dim_unit
 * @param string          $weight_unit
 * @param WC_Boxpack      $WC_Boxpack
 * @param WC_Shipping_UPS $WC_Shipping_UPS
 *
 * @return mixed
 */
function tt_ups_maybe_use_separate_package_for_product( $skip, $cart_item_key, $cart_item_data, $dim_unit, $weight_unit, $WC_Boxpack, $WC_Shipping_UPS ) {
	if ( $skip ) return $skip;
	
	// Get some cart item data
	$product_id = $cart_item_data['product_id'];
	$variation_id = $cart_item_data['variation_id'];
	$quantity = max( 1, $cart_item_data['quantity'] );
	
	// Abort unless "_packed_separately" is enabled for the product (for variations, checks the parent product)
	if ( ! get_post_meta( $product_id, '_packed_separately', true ) ) return $skip;
	
	// Get dimensions and value for packing
	$length = number_format( wc_get_dimension( $cart_item_data['data']->get_length(), strtolower( $dim_unit ) ), 2, '.', '' );
	$width = number_format( wc_get_dimension( $cart_item_data['data']->get_width(), strtolower( $dim_unit ) ), 2, '.', '' );
	$height = number_format( wc_get_dimension( $cart_item_data['data']->get_height(), strtolower( $dim_unit ) ), 2, '.', '' );
	$weight = number_format( wc_get_weight( $cart_item_data['data']->get_weight(), strtolower( $weight_unit ) ), 2, '.', '' );
	$value = $cart_item_data['data']->get_price();
	
	// Add a box (or multiple boxes if qty more than 1)
	for ( $i = 1; $i <= $quantity; $i++ ) {
		$WC_Boxpack->add_package( $length, $width, $height, $weight, $value );
		
		// Report some debugging info
		$WC_Shipping_UPS->debug( '['. $i .'] Self-packed item added:<br><pre>' . print_r(array('product_id' => $product_id, 'variation_id' => $variation_id, 'length' => $length, 'width' => $width, 'height' => $height, 'weight' => $weight, 'value' => $value), true) );
	}
	
	return true;
}
add_filter( 'woocommerce_shipping_ups_skip_packing', 'tt_ups_maybe_use_separate_package_for_product', 10, 7 );


/**
 * Add a checkbox below Dimensions on shipping tab of products to allow enabling the "Packed Separately" option.
 */
function tt_packed_separately_field() {
	global $post;
	
	// Display checkbox below dimensions field
	woocommerce_wp_checkbox(
		array(
			'id'                => '_packed_separately',
			'wrapper_class'     => 'show_if_simple show_if_variable hide_if_external hide_if_grouped hide_if_virtual',
			'label'             => __( 'Packed Separately', 'terratech' ),
			'description'       => __( 'Product is already boxed and should not be packed with other products (UPS Only).', 'terratech' )
		)
	);
}
add_action( 'woocommerce_product_options_dimensions', 'tt_packed_separately_field' );

/**
 * Save "_packed_separately" metadata to the product
 *
 * @param $post_id
 */
function tt_save_packed_separately_field( $post_id ) {
	$checked = isset( $_POST['_packed_separately'] ) ? 'yes': '';
	update_post_meta( $post_id, '_packed_separately', $checked );
}
add_action( 'woocommerce_process_product_meta', 'tt_save_packed_separately_field' );