The addition of the ability to accept and manually process credit card payments sounds great.
My question is how or what is might be needed to secure the collection and storage of user credit card and authorization information for manual processing of payment with DTRegister ?
You should have a SSL certificate loaded for your domain so the form itself is secure. Then install/enable our SSL plugin. Beyond that, there's nothing else you need to do. When the data is collected, the card number is stored using 256-bit encryption. Then when you view the record in the backend, the card number will be decrypted so you can see payment info. You can of course secure the joomla admin itself also if you wanted.
How can I delete the credit card details alltogether?
For maximum security / pci compliance there should be some way to delete the details from the database after processing.
There is no method of removing the stored data outside of just manually removing it from the database. It is stored in jos_dtregister_cards and you'd be able to determine which data to drop as the name and such are visible. Only actual card data is encrypted.
We'll look into adding a method of deleting the stored data from the DT Reg admin.