$points=
$grand_total/100;

$conn->query("
INSERT INTO customer_loyalty(
customer_id,
invoice_id,
points,
transaction_date
)
VALUES(
'$customer_id',
'$invoice_id',
'$points',
NOW()
)
");

$conn->query("
UPDATE customers
SET loyalty_points=
loyalty_points+$points
WHERE id='$customer_id'
");