"Does that mean any return efiled today thru Monday will have a late payment?"

Extension to file does not extend the due date for making tax payment, which was April 18.  Late payment penalty will be assessed if the taxpayer has not paid in at least 90% of the taxes owed.

If your client qualified for automatic 2-month extension for residing overseas on the original due date, however, late payment penalty will not kick in until June 15 although interest will still accrue on any balance due from April 18.