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

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?

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