Learn Dutch: Lippenstift koopt ze altijd veel. - Lipstick, she always buys a lot.

This is an example of a discontinuous noun phrase, often called split topicalization. Instead of using the standard order Ze koopt altijd veel lippenstift, the speaker pulls the bare noun (lippenstift) to the front to make it the topic of the sentence, while the quantifier (veel) gets left behind at the end of the middle field.

I learned we need to use er with separated quantities. Why isn't it Lippenstift koopt ze er altijd veel?

You are thinking of the quantitative er, which is required when referring back to countable nouns (for example: Boeken koopt ze er veel). However, when a noun is used as an uncountable mass noun, like geld (money) or lippenstift (lipstick in general), the er is usually left out when paired with quantifiers like veel or weinig.

Why is the word order koopt ze instead of ze koopt?

Because Lippenstift is placed at the front, it occupies the first position in the sentence. Dutch follows a strict verb-second (V2) rule in main clauses, so the verb (koopt) must take the second slot. This forces the subject (ze) to move immediately after the verb, causing inversion.

Does fronting the word lippenstift change the tone of the sentence?