Oft Traders assume that the customs value is based on the invoice price, which is not necessarily true. The value to be used is the transaction value including all costs that are on the goods at the time of the transfer (to the EU). If the Incoterm is CIF, then the invoice amount is according to the customs value, except r there are still costs such as molds or so, which are put into separate bill. For example, what can be considered for costs can be found on the second page of the old form 0464 (can be found on Zoll.de). For example, if the delivery term contains less than CIF, FOB, freight costs must be fully paid from the port of shipment to the point of shipment (to the EU) and duty paid at full duty. Should the delivery condition go further than, for example, CIF, DAP, may or may not, deduct the costs from the point of shipment to the place of delivery. Somewhat more complicated it becomes with DDP, because then also the customs from the customs value can be calculated out – a popular task for trainees 

The import turnover tax, on the other hand, is calculated on the basis of the CIF value mentioned plus the customs duties (yes, customs duties are taxed) and all costs from the point of sale to the place of delivery. However, the so-called follow-up costs can not be offset here if the biller (forwarder) charges these costs with VAT. Otherwise, the amount would be taxed twice what you only know of fuels.
If you can not find the 0464 form but would like to have it, feel free to contact us. Also, if you want a small tax calculator in Excel, or just have questions on the topic, like to comment or send PN.