Unterstuetzt.

Just as a general comment, all similar entries with shop may need to be marked [chiefly Brit.], and perhaps also the ones with assistant. 

One traditional AE term was sales clerk / salesclerk; the sexist version was saleslady (possibly also salesgirl?), and the modern inclusive term is salesperson.

It's possible that the employees of large chain stores may now be called (sales) assistants in in-house jargon, or as their official job title. But I'm skeptical that most customers in modern AE would say 'See if you can find an assistant to ask' or 'The assistant wouldn't stop talking on the phone to wait on me.' So my impression is that that term in this sense may be chiefly BE as well, at least in the sense of a small independent store or local shop.

As far as this specific pair goes, it does indeed seem like you could just pair shopgirl with Ladenmädchen, and save assistant for a less sexist translation like Verkäufer|in.

Das Ladenmädchen sollte als veraltet gekennzeichnet werden - ich habe den Begriff heute zum ersten Mal überhaupt gehört (hätte ihn auch nicht unbedingt mit Verkaufspersonal in Zusammenhang gebracht) und daraufhin erstmal nachgeschlagen. Die Hauptzeit, in der dieses Wort verwendet wurde, liegt wohl zwischen 1900 und 1930.