The way it works is this:
First year you donate at the silver level - at the end of the first year, it goes away and if you want it again, you donate at the silver level and it is there for the next year. If you don't donate the next, it goes away.
So, if you want more storage, donate at the gold level.
I hope that helps. There will be some overlap, so you will have a chance to update your donation before it goes away. We will be posting this in the near future for members.