missing e-file payment

My son, a minor, filled taxes for the first time this year.  We submitted them on time, but just barely, on the 18th of April, thus we paid the amount he owed using my credit card.  I've verified the charge on my credit card and see that it was made on the 18th of April.

My son just got a letter from the IRS stating that he owed them the amount due per the tax forms he filed.  We completed his taxes using the Turbo Tax application on a Mac and e-filed them via Turbo Tax.  My son has emails from Turbo Tax that the return was filed and accepted.  We can't find anything about the payment.

Does anyone know if there's something else we needed to do or how we can get a receipt to show the IRS he mad this payment?