I participated in the Bonus offer for PartyPoker and I cleared it and I chose the books and I've waited for the Books to arrive and now

I got a mail from German customs stating that the required invoice was not on the outside of the package, and that's why they put it up at the customs,

Do I have to pay taxes on those books now?

The invoice stating that it's a gift and the total Value is in the mail I got from the customs department.

Is that invoice with the sender/recipient/value enough for the customs department or do I need something else?

I heard some people were printing out the Bonus Page and went to the customs with it but I'm not sure what I should print out there?

In some countries books are duty free. Ask if that is the case for your country. We do mark the shipments as a gift and value @ $24. The shipping order (not enclosed with the shipment) shows retail price and a 100% discount. I can send that over as a PDF if the helps.

If you need to justify the value, it was a gift from the publisher and that was the cost of printing.

I went there and picked it up today without having to pay anything