Making Payment at Post Office with PC-Konto No.

Today I wanted to make a small payment (donation) to an address which only provided a PC-Konto number. A name and address was also provided.
I was more than surprised when the woman at the PO payment desk said that it is impossible without the IBAN. She fiddled around with her computer but insisted she could not make the payment. Surely the PC-Konto No. should suffice?

If you have the IBAN, what do you need the post office for?

They don’t have the IBAN.

Having said that it should be possible to go online and find the IBAN number from the account number.