add_weight_product_addon.php

<?php
/**
 * Add a custom weight parameter to add-on field settings
 */
function prefix_add_option_param( $option_count, $group_id, $item_key, $item, $key ) {
	$name = '_product_extra_groups_' . esc_attr( $group_id ) . '_' . esc_attr( $item_key ) . '[field_options][' . esc_attr( $option_count ) . ']';
	$weight = isset( $item['field_options'][esc_attr( $key )]['weight'] ) ? $item['field_options'][esc_attr( $key )]['weight'] : '';
	?>
	<td class="pewc-option-extra">
		<input type="number" class="pewc-field-option-weight" name="<?php echo $name; ?>[weight]" value="<?php echo esc_attr( $weight ); ?>" step="0.01">
	</td>
<?php }
add_action( 'pewc_after_option_params', 'prefix_add_option_param', 10, 5 );

/**
 * Add the new weight param to the cart item data
 */
function prefix_end_add_cart_item_data( $cart_item_data, $item, $group_id, $field_id, $value ) {

	if( ! empty( $item['field_options'] ) ) {

		// Check if $value was originally an array
		if( strpos( $value, ' | ' ) !== false ) {
			$value_arr = explode( ' | ', $value );
		} else {
			$value_arr = array( $value );
		}

		foreach( $value_arr as $value ) {

			foreach( $item['field_options'] as $option ) {

				$option_len = strlen( $option['value'] );
				if( substr( $value, 0, $option_len ) == $option['value'] ) {

					$weight = isset( $cart_item_data['product_extras']['weight'] ) ? $cart_item_data['product_extras']['weight'] : 0;
					$weight += floatval( $option['weight'] );
					$cart_item_data['product_extras']['weight'] = $weight;

				}

			}

		}

	}

	return $cart_item_data;

}
add_filter( 'pewc_filter_end_add_cart_item_data', 'prefix_end_add_cart_item_data', 10, 5 );

// Update the weight
function prefix_get_item_data( $item_data, $cart_item ) {

	if( ! empty( $cart_item['product_extras']['weight'] ) ) {
		$item_weight = $cart_item['data']->get_weight();
		$item_weight = $cart_item['product_extras']['weight'];
		$item_data[] = array(
		    'key'       => __('Weight', 'woocommerce'),
		    'value'     => $item_weight,
		    'display'   => $item_weight . ' ' . get_option('woocommerce_weight_unit')
		);
	}

	return $item_data;
}
add_filter( 'woocommerce_get_item_data', 'prefix_get_item_data', 10, 2 );