Ok, I didn't show this one here because, at 1850, it's not my oldest. But now I just have to because this stamp less folded letter was ALSO sent to Mr D.S. Kennedy of New York!

Purchased at the recent CSDA (Canadian Stamp Dealers' Association) show in (well just outside of) Toronto, this cover was sent to Mr Kennedy, a prominent NY banker on behalf of a gentleman named T. R. Merritt.
Thomas Rodman Merritt was the son of William Hamilton Merritt, the man who founded the first Welland Canal in 1837, which bypassed the Niagara River and Falls and helped improve trade in the region. Merritt Jr continued in business with his father, and his home still stands on the banks of what was to be the second Welland Canal (there have been four) and today houses a the local art gallery, Rodman Hall.
I was born and raised in St Catharines, and my family were settlers in the area after the American Revolution (Loyalists we were), so this interest in early Niagara covers has been brewing for a while.
This letter was postmarked Dec 10 1850 in St. Catherines (I've noticed that early cancellations were misspelled with the 'er' rather than the 'ar' I was raised to use). It reached its destination in New York via Queenston (cancelled on the back in black on Dec 11), Lewiston NY, seen cancelled on the front in red, then on to the Big Apple (though I guess it wasn't called that at that time). It seems to be business correspondence, though for the amount of paper that is folded here, there is very little writing. Makes me wonder if some money wasn't included, though there is no indication that it was a money letter. The sender, whose name I can't make out, appears to be an agent for the Bank of Montreal (coincidently, my employer, at the moment!) Sums of money are discussed, but it's all a mystery to me at this point.
10d. postage was paid, but PAID has been crossed out, the 4 1/2p added. Postage rates during this period are also a mystery to me.
If the book you site is a good reference for postal rates at the time, it's one I will have to track down for my library!