Hi Jens, in answer to your questions:
1. No, ClassiPress does not receive any kind of invoice when an ad is placed nor are there any invoices generated by the theme. The admin keeps track of purchases through the "
transactions" page of the ClassiPress admin.
2. The theme is not encrypted, so you are free to make such changes if you wish. AppThemes does not offer customization services, however we do have a
certified partners page where you can find developers for hire. If you make contact directly with an outline of your requirements they can provide a quote. Thanks.