I've read over the link that Brian posted - and while it makes sense, I see a bit of a problem.
The Documentation notes say :
And while this is fine, what if you have a group of products (for example, shared hosting plans) that all use the same configurable options. And you configurable options are things like :
Additional Disk Space
* Add 1 Gb Disk Space
* Add 5 Gb Disk Space
* Add 1 Gb/Month Traffic
* Add 5 Gb/Month Traffic
The issue here is that with the way the documentation reads, you would need to have a set of configurable options for EACH hosting plan, to allow it to increase the disk/bandwidth amounts correctly.
If hosting plan 1 has a disk limit of 1Gb, and you puchase and Additional 1Gb, your configurable Option would have the "Add 1Gb Disk space" as setting the disk space to 2Gb.
but if you have a hosting plan 2 with a disk limit of 1.5Gb, and the user purchases the "Add 1Gb" it would then only increase the disk space from 1.5, to 2Gb - because of the limit set.
So, the question is :
Can we have the configuration options to "add to" and "remove from" the soft limit on the product, rather than setting arbitrary limits? for example - have the limit as a "+1Gb"... that way, if the user downgrades, and removes the additional disk space, they return to the original limits of the plan?