If you use (prepaid VAS) validity extension, money is deducted from your prepaid main balance -- JUST LIKE money is deducted from your prepaid main balance when you buy any prepaid data plan or buy any prepaid voice/IDD plan etc.
Since you PAY MONEY to buy the prepaid validity extension -- the validity extension WILL STACK to the current prepaid's expiry.
If your prepaid expires on 2019-04-01, and you buy the 90day $1.40 validity extension on 2019-03-15:
90 days will be added to prepaid expiry 2019-04-01.
Then the new prepaid expiry will be 2019-06-30.
On the other hand,
Any validity you get from any prepaid top up, DOES NOT STACK, e.g.:
If you top up $18 to your prepaid, you get 180days validity.
If you top up $10 to your prepaid, you get 120days validity etc, etc.
This means:
Those 180day/120day validity you get from prepaid top up -- is calculated from the DAY you do the prepaid top up.