Fedex — Payment already made

I am not pleased with the way Fedex operates they sent me an invoice of 12.25 euro which I made to AIB electronically on the 4th of February but I have been receiving letters telling me that they did not receive the money and I checked with my bank and a payment was made to this AIB Iban [protected]. I tried emailing them the screenshot of the payment but they seem to have no reply email address which makes it difficult to give them the proof.Abd now they are telling me they will send debt collectors for the payment which I made and have proof.