From the monthly pricing section of the website...
FROM 07 MAY 2014 THIS IS HOW ADD-ONS WILL WORK
Add-Ons are no longer pro-rated. You will be charged the full amount for your Add-On and receive the full included quota at the time of purchase. You can add any of these Monthly Add-Ons to your active Monthly Combo at any stage. The Add-On will auto-renew every 30 days from purchase if your Combo has auto renewed, is still active and you have enough credit to buy the Add-On.
If you choose to Change Combo and move to a different Monthly Combo, your Add-On will remain active and will be set to renew on its planned renewal date.
If you cancelled your Combo, or it failed to renew due to not enough credit, your Add-On will also cancel but you’ll keep any remaining quota from your Add-on until its expiry date.
If you want to purchase the same Monthly Add-On more than once during your Combo’s cycle, you'll need to cancel the existing Add-On first and then purchase a new one using the Skinny Mobile Menu or our website. You’ll keep any quota remaining from the original Add-On until its expiry date. Your Add-On renewal date will change to 30 days from the time you purchase the new Add-On.