I installed Canada Post 1.5.3 mod with zc 1.5.4. I found 3 core files from zc 1.5.4 that were different from zc 1.5.3, so I merged only the Canada Post code sections on those files, by first comparing the the Canada Post mod to the zc 1.5.3 files, then merging only those differences into the zc 1.5.4 files.
admin/incl/mod/copy_to_confirm
admin/incl/mod/update_product
incl/classes/shopping_cart

The rest of the files were straight merges.

Has been working for me.