Okay, here is the scoop on the overnight envelope as I understand it.

They ARE NOT to charge us for postage.

The SOS corporate account number is on the label and that account gets
billed for the postage.

I called our local Post Office just now and the lady (different person from
yesterday) was apologetic that I was charged and will issue a refund. 

So even though my label from March was filled in with the postage, don't
believe we paid it at that time but that the SOS office would have been