After a few clients getting this notice and them all calling upset that they paid we tried the call the IRS route. Obviously that didn't get them anywhere so we are going to try this:

send a copy of the cancelled check or print out of the online payment to the IRS with the payment voucher request to the address they want the payment sent to. Hopefully when the IRS opens it up and sees no payment they review what was sent and fix it. Fingers crossed.