If you paid digitally using your a credit card or the method you mentioned above, then make sure to check the SPAM folder of your email account. Sometimes, SPAM filters catch those emails and they're moved to that folder instead.
If there is no email there with the registration number, visit the following website and send an email to the developer or call them if you can: http://www.softwarenetz.de/kontakt.php
This is the only way to obtain support when it's something related to payment and registration numbers.
