For all of the plans that are listed "per month", we will calculate from the time you purchased the addon to the time your current package is up for renewal, and you will only be billed for the difference.
For example, lets say you have a yearly package. 4 Months from when you started you decide you want to buy 1 GB of extra disk space. Since there are 8 more months left on your package, you will only pay for 8 months worth of extra disk space up front, or $8.
*SSL Certificates can not be broken up into months and are only billed at the yearly rate.