Earlier quoted context omitted.
"c/o" is fine, I've used that before. [edit – wrong:] The German-language equivalent (that most young people won't even recognize anymore) is "z. Hd." ("zu Händen" – "to the hands of").
The German equivalent would be p. Adr. (although I haver never seen that much in the wild). z. Hd. is used in the opposite way, i.e. you write the actual recipient after it, not the address provider.
I haven't actually seen "p. Adr." ever in my life.