Learn Portuguese (Portugal): Ou aceitas a oferta ou perdes a vantagem. - Either you accept the offer or you lose the advantage.

The pair ou ... ou is a correlative conjunction that translates to "either ... or". Just like in English, using them together sets up a strict choice between two options. The first ou acts as "either", and the second acts as "or".

Could I use quer ... quer here instead of ou ... ou?

Not without changing the meaning completely. While both are correlative conjunctions, ou ... ou gives a choice ("either ... or"), whereas quer ... quer means the outcome happens regardless ("whether ... or"). Because this sentence presents an ultimatum, ou ... ou is the only one that makes sense here.

Where is the word for "you" in this sentence?

It is hidden in the verbs! Both aceitas and perdes are conjugated in the tu (informal "you") form, which typically ends in an -s in the present tense. Because the verb endings make the subject perfectly clear, European Portuguese usually drops the pronoun tu.

How would I say this to someone in a formal setting?

To make this formal using você, you just need to drop the -s from both verbs. The sentence would become: Ou aceita a oferta ou perde a vantagem.