To further clarify, this was not my error.  The client is in the 28% tax bracket and the 2018 worksheet for that tax bracket was not finalized until sometime in May, 2019, well after the tax return was filed.  The client was not accessed penalties, just tax and interest.  People have responded that the IRS is mandated by law to access interest, regardless of the cause and I thank them for their response.  I do not plan on paying the interest for this client nor are they asking me to.