OK - I figured out the answer to my questions.
Canada Post Module with Lettermail 1.3.8 will install on Zencart 1.3.7. following the same txt instructions as for installing onZencart 1.3.8.
And to my question about lettermail codes...on the edit screen of the module, if you leave the USA rates BLANK then it will not offer that service and will bring up the Small Packet rate as appropriate (if you have that enabled in your Canada Post profile).
I've tested this on the local copy of my site. I'll test it a few times more before loading onto my live site.
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