I understand why the bank wont give the letter. If it is a scam and you lose the money you can hold the bank liable.
They should be happy with a letter from you.
The bank is attempting to distance itself and not be held liable.
What guarantees do you have from the polish bank that it is not a scam?
The bank is not going to take the risk and I suspect it is a policy.
The receipt that you have sent the money should be enough for the polish bank to confirm that the money has been sent.
A letter from you should also be more than enough.
I believe the bank is not sending any letters to reduce its exposure to claims if it is a scam and you lose your money.
I have send money all over the world and never had anyone seek confirmation in a letter from the bank that is was sent. the documentation off the TT produced by the bank and signed by me has always been enough to prove the transfer.
It is a detailed note showing the exchange rate the accounts it came from and sent to and the names of the parties and you as the customer sign to confirm you authorise it.
If the polish bank refuses to accept this official document I would have concerns and seek my money back.
What the polish bank wants is not standard practice.
I can see why Bendigo bank is not sending the letter, it exposes them to liability from you.