@joerg_rw:
Since you have the e-mail adresses I think it would be best to send an "official" e-mail to actually ask for the money.
There have been multiple risk part warnings in this thread and not all of your customers might be watching this thread so closely.

Sending an e-mail would show everyone this time you're serious.
It should contain the info about prices, "donation" handling and e-mail/paypal you gave in your last post plus a realistic deadline (people might be on holiday).