Setting a new Line Item Default

functions.php

<?php // don't include this line in your functions.php, since it already starts with it.

function _set_new_default_si_type( $types = array() ) {
	unset( $types['service'] ); // remove the type you want to make a default
	return array_merge( array( 'service' => __( 'Service', 'sprout-invoices' ) ), $types ); // add it back at the top of the list.
}
add_filter( 'si_line_item_types', '_set_new_default_si_type' );