we're in a similar situation to the OP, again with Stuttgart city authorities regarding payment of a speeding fine. We have taken the advice of other posts on this thread & went to try & pay via our local Halifax branch, When the teller tried to process the transaction Stuttgart City had failed to provide any bank details except the IBAN number and so were unable to pay the fine. Halifax are going to charge us £19.50 for doing this at the local branch!
I have sent 2 emails to Stuttgart council in both English with a German translation underneath and am currently waiting to see if they will accept a credit card payment via email.
In addition, from what I could translate, they want payment within a week of the date of the letter, yet it wasn't posted until 4 days after the letter was dated!
Any suggestions?