Performance Credit Question

I had a few questions about performance credits that seller support couldn’t answer clearly. If anyone has any insight thank you so much.

Are we charged storage limit increase fee and regular storage fees?

For example I have 1020 cubic ft of extra storage that I will be paying for in October. I got any email saying the I got the same amount in September in advance for free per an email from SS but would I pay storage on that?

For extra storage that I requested, if I end up getting performance credits for it by selling my items out, I won’t get charged the extra storage I requested or actual storage cost correct?

I have more storage in previous months so I will carry over some units, how would they calculate the revenue on 1000 Ft storage if I had 300-400 additional the previous months?

Like which date would it start for me to count sales on units to go towards my performance credits if I have 1000 extra cubic feet?

TIA!