[JMTQp8lwXL]

Many local governments publish shapefiles (geojson) that have address information. You could parse out addresses and have a fairly simple free option. Since this data is based on property taxes, these data sets aren't going to be missing parcels (or the tax collector would be missing potential revenue).

[smnrchrds]

That still leaves out a lot: unincorporated lands, First Nation reserves, small settlement where all the mail goes to a post office instead of individual buildings, etc.

Also, one of the main benefits from Canada Post API is that it gives you the canonical address Canada Post uses to deliver packages. I don't think this always overlaps with addresses municipalities use to assess taxes. For example, I can imagine for an apartment building belonging to a real estate management company, there would be a single entry in the municipality database (because there is only one owner who needs to pay taxes), but one entry per unit in Canada Post database.

[JMTQp8lwXL]

Yeah, my initial suggestion likely misses some corner cases. One case I am fairly confident it covers is (not sure if correct term) mutual interest land ownernship (aka, condos). Each layer has the same shape, but they're stacked one top another; each layer is a billable portion of the interest (e.g., a single condo on the shared land).

A poor man's approach could be to query the publicly available data sets, and use that data when there's match. Then, only pay for queries that fall into the edge cases.