1. No notification/letter (unless it's via cheque)
2. You may call them....
3. Maybe you actually owe them more then you thought ?

Normally is due to you wrongly declare or put higher than max amount allowed on certain parts.

My advise is always:
(1) Are you very clean and always declare ? Be prepared they pick you up and ask you bring all receipts, etc. for audit. If clear and nothing to hide, no problem. Else kantoi.

(2) It is your right to know, and they must be able to show you their calculation on the new amount. Just call up or email, set an appointment, location and go bang table with them. But ... be prepared for #1 above.

(3) Otherwise, for that RM100+, my normal advise is, just move on with your life. Otherwise, get audit, waste time, money, petrol, parking, summon for park at roadside, etc. At the end, may get fine/penalty cause they found out more irregularities or non-declare or under-declare from other years ... kantoi.

If you are NOT paying Income Tax under Installment, then just put 99 (it's written on your physical form and M2U also defaults to 99)