I don't think it's necessary in the translation to include a rephrasing in parentheses. Considering all the different countries in which English is spoken and the corresponsing variety of postal systems, it seems to me that the author is just clarifying the point by offering an alternative wording of the same thing.

My Pons dictionary, for example, supplies EINSCHREIBEN = recorded delivery (Brit) & certified letter/parcel (US), and I'm sure other variants in the English-speaking world are possible.