While you're at it, explain to him how to answer the client who calls in September and says he wants to change the amount, or the account number.  

A simple phone call cancels the scheduled payments, then you can use IRS Direct Pay to make any changes and reschedule any remaining payments.

"Simple" and "phone call" are generally not found in the same sentence when discussing IRS, but I'll take your word for it.  How does the person at the other end of the line know it's the taxpayer?  (And I hope the preparer isn't allowed to do it.)

Helped a client a couple years ago whose family member died in early September.  The scheduled payment for September 15 was rejected by the bank.  IRS charged a penalty of $40.  Wasn't worth a simple phone call to try to get it back.  

Ive done it numerous times for clients, its not a problem.

What's a reasonable fee to bill them for that?  And do you ever go on vacation so you're not around when they ask?

Its literally 10 minutes out of my life, Im not going to charge them for it, I may bump up their bill a little more than usual the following year though.

Vacations stress me out, packing, traveling, unpacking, UGH!    Im never out of town more than a few days at a time...its never been an issue.