Set up auto-renew from credit card to ensure you are only charged for your rate plan every 30 or 365 days.
You can purchase multiple add-ons at the same time. They create a line-up allowing for immediate use of the next when the current one expires or depletes. The 30-day service period starts when the add-on comes into effect, not when it was purchased.
Stacking rate plans
You can purchase multiple rate plans at the same time. Note they must be different rate plans. Each is valid for 30 days or 365 days (depending on the rate plan) and the next will only come into effect when the previous one expires. This creates a line-up allowing immediate use from the next rate plan when the first expires or depletes.
If the first rate plan runs out before its 30-day expiry date, the second rate plan takes effect immediately, and the expiry date is 30 days from that moment.
To increase a talk, text or data allotment, you can combine rate plans and add-ons. Each will expire based on the date it was purchased, not when it takes effect.
So if the rate plan allotment runs out before it expires, the add-on will take effect immediately, but the add-on will have the same expiry date from when it was purchased.