Thanks for the suggestions of alternative places to register .de domains, but both suggestions are more expensive than the one I found.
I like dd24.net in general, particularly their wide choice, but their prices are not the cheapest, but not a rip-off either. Their €10.25 fee is more than Host Support's fee of €5.90.
Idotz.net are even more expensive at $19.95, which is around 3 times the price of Host Support.
I can use a friend's address in Germany for the admin contact, so that is not a problem for me.