Though I haven't checked, I would think (and treat it as such) that the metered sticker affixed to the packet by the USPS Clerk is somewhat of a legal document that notes their total fee in attempting to deliver. I do not recall ever having to pay any additional postage on anything I've ever mailed with a metered sticker that has been bumped. I think it's a done deal; they're telling you how much it will cost; you're paying them; they follow thru with documentation, and I believe that is all that's needed.

JB