ADD-ON packages are advertised as being valid for a period of 30-days but in practice they become virtually unusable as soon as the current billing period expires.
This is because they are always applied After the current months allowance.
This is fair and correct for the month in which the add-on is purchased but is grossly unfair when it rolls over to the next billing period.
I can only use any of the remaining addon allowance if I use ALL of my monthly allowance prior to the expiration date of the addon and if I do achieve that my remaining allowance will plummet to zero when the addon does expire risking unexpected out-of-plan usage charges or forcing me to purchase yet another addon to cover the remainder of the month.
The correct and fair approach would be to treat the rollover of an addon the same as a data rollover and to use it prior to using the current months allowance.
This would allow me to use all of the remaining allowance and still have the whole of the current months allowance to use after the add-on has been used up or expired.
The simple rule should be that the allowance (add-on, data rollover, monthly allowance) with the earliest expiry time should be used first. This would allow users to gain the maximum benefit from all their allowances,