I just called Canada Post's help line for their "sell online" accounts, to see if they might be able to help with getting Lettermail quotes to show up or see what the error is on their end.

The person I spoke with said that they don't offer quotes for lettermail, to show directly on sites. I let them know that I do have it working for USA & INTL quotes & that those quotes are showing using this module and their "table rates", but he still said that there is no such feature that they offer. So they must just not be aware of how this module works or something.

Anyway, they unfortunately couldn't help, but he did suggest to just set up a "flat rate" shipping option that would cover the amount charged for any Lettermail shipments within Canada. So if I'm unable to get the Canadian Lettermail quotes to work with this module, i may try that.
I'm also wondering now if i should try to set it up through the "Table Rates" shipping module that's already included in zencart. I'm thinking that maybe I can set that up for the same table rates, and put the "Shipping Zone" to Canada (if it even allows that) if that will maybe work to provide Lettermail quotes for my Canadian customers.
I may try that now - I'll update here with the results, in case anyone else is having the same issue as me.