Add new item type and replace column title

functions.php

<?php // don't include this in your functions.php


// add new item type (working)

function add_sizeproduct_line_item_type( $types = array() ) {
	$types = array_merge( $types, array( 'sizeproduct' => __( 'Size Product' ) ) );
	return $types;
}
add_filter( 'si_line_item_types',  'add_sizeproduct_line_item_type' );


// replace new item types column 'rate' with 'Per SQM' (not working)

function add_persqm( $columns = array(), $type = 'sizeproduct' )  {
	if ( 'sizeproduct' !== $type ) {
		return $columns;
	}
	$columns = array(
		'_id' => array(
				'type' => 'hidden',
				'value' => mt_rand(),
				'weight' => 0,
			),
		'desc' => array(
				'label' => __( 'Size', 'sprout-invoices' ),
				'type' => 'textarea',
				'calc' => false,
				'hide_if_parent' => false,
				'weight' => 1,
			),
		'sku' => array(
				'type' => 'hidden',
				'placeholder' => '',
				'calc' => false,
				'numeric' => false,
				'weight' => 3,
			),
		'rate' => array(
				'label' => __( 'per', 'sprout-invoices' ),
				'type' => 'small-input',
				'placeholder' => '120',
				'calc' => false,
				'hide_if_parent' => true,
				'weight' => 5,
			),
		'qty' => array(
				'label' => __( 'sqm', 'sprout-invoices' ),
				'type' => 'small-input',
				'placeholder' => 1,
				'calc' => true,
				'hide_if_parent' => true,
				'weight' => 10,
			),
		'tax' => array(
				'label' => sprintf( '&#37; <span class="helptip" title="%s"></span>', __( 'A percentage adjustment per line item, i.e. tax or discount', 'sprout-invoices' ) ),
				'type' => 'small-input',
				'placeholder' => 0,
				'calc' => false,
				'hide_if_parent' => true,
				'weight' => 15,
			),
		'total' => array(
				'label' => __( 'Amount', 'sprout-invoices' ),
				'type' => 'total',
				'placeholder' => sa_get_formatted_money( 0 ),
				'calc' => true,
				'hide_if_parent' => false,
				'weight' => 50,
			),
	);
	return $columns;
}
add_filter( 'si_line_item_columns', 'add_persqm', 10, 2 );