Vouchers to help your client to keep track and to flag the IRS to apply physical payments or Online payments through EFTPS to the right Period.

The vouchers are pretty much moot, when this is Pay by Keeping my overage. The return Filing Date is when the IRS considers they have the amount to apply, because you stated on the return to apply to the next tax year.

And of course, the IRS cannot apply anything to "June" if the return is not even filed until July. The whole point of "let it ride" = Hey, I've already partially paid for the new year. So, either you Filed, and there were funds credited; or, you have not yet filed, so you need to make a payment in consideration that the IRS wants some money sooner.