Is it the same at all Post Offices or just this one in Pocono Summit?  They will not except a prepaid mailing if was paid for a day or two before I could drop it off.

Technically it is stale postage and needs a zero strip added to update the post mark. I think that is why PBS changed the printable postage to allow the post mark to be in the future.  I've never tested my PO on the issue, but I think there are a few places that are more forgiving.