I wouldnt put anything under the wiper, as if they ignore it and drive off and have an accident someone would be liable. Thats the reason parking tickets are not placed on windscreens.
I would suggest that a Business card placed in the drivers window seal near the door handle would be better.
With something like...
Did you know that there is a Audi A2 owners club community that has lots of great free and helpful information on this future classic car? Feel free to join us online at...
With the web address...