how much would i have to pay at german customs?
Not German, but for the EU you calculate: price you paid of item itself +
duty (zoll?) according to TARIC. Add together item price with duty applied + shipping cost, then add
sales tax (MwSt, VAT etc.) on top, and yes the sales tax is compound on top of duty.
You may expect a customs processing fee, but for Germans it is usually zero or a nominal fee, since for the time being international law is followed and courier agents are not able to charge extra fees to customers just because they feel like grabbing an extra profit from your item.
Fortunately the duty on computer parts and accessories is zero. For Germans 19% sales tax, so it's easy and you just need total price paid at Massdrop checkout × 1.19, plus maybe a few € processing fee.