My two favorite "Over the Top" covers are both registered covers, both from Humboldt County, California
The first cover is from Fruitland, posted Apr 8,1896, it is franked with 10 First Bureau 1 cent (264, I think, haven't checked watermark) pay the 2c first class rate and the 8c registry fee, addressed to Humboldt County's tax man, the back has nothing but pencil notes.
The other "Over the Top" cover comes from Martin's Ferry, up in the north-east corner of the county and has a bit of a mystery. The cover is misshaped from the contents and the 10 1882 re-engraved banknotes pasted on the back that held the cover in that shape. This cover bears .31 cents in postage and Registry fee.
Here's the mystery, the manuscript postmark doesn't match the only other known Martin's Ferry manuscript postmark.
First my cover;
Martin's Ferry opened Mar 18 1874 to Oct 1 1891 name changed to Weitchpec.
And the other known Martin's Ferry cover, image taken from one of Schuler Rumsey's auctions.
These two covers were posted about 3 months apart, from a Class 4 post office, in a less than desirable location (northeast corner of Hoopa Indian Res on a big bend of the Klamath River)
I'm guessing that Mrs. Martin took care of my cover while Mr. Martin was on the river earning their name.
"Pull Lemual, Pull!