One thing you will need to consider: Do you have a means of checking the names of the recipients against the OFAC list? For cross-border ACH payments, that responsibility falls on the ODFI.
Although the situation you are describing implies that the recipient will be your own customer, once you are set up to process ACH payments bound for Canada it is technically possible that other customers for whom you originate ACH could, without your advance knowledge, include cross-border payments in their files.