On 9/18 we rec'd a cp503 that we owe passed due taxes. We have paid on time

[DoninGA]

If you paid the taxes owed and the payment was processed by the IRS then you should have proof of payment either as a cancelled check, bank or credit statement showing the Payee and Payment to the IRS.

If you paid the taxes owed through the IRS payment website then you should have a confirmation number from the IRS.

Do you have the proof of payment?

[VolvoGirl]

How did you pay it?   Did you mail a check with a 1040-V Voucher.  Or did you use a 1040-ES slip?