Add Payment ID to Sales API

function kjm_add_payment_id_to_sales_endpoint( $sales ) {

	foreach ( $sales['sales'] as $sale => $data ) {

		// If sequential order numbering is on, the ID key will be a string

		$payment = ( is_string( $data['ID'] ) ) ? edd_get_payment_by('payment_number', $data['ID']) : $data['ID'];

		$payment_id = ( is_object( $payment ) ) ? $payment->ID : $payment;

		$sales['sales'][$sale]['payment_id'] = $payment_id;

	}

	return $sales;

}

add_filter( 'edd_api_sales', 'kjm_add_payment_id_to_sales_endpoint' );