Aceita-se Pix, mas não se aceita cheque.

No. Here se is not reflexive, so it does not mean something like accepts itself.

In this sentence, se is used to create a kind of impersonal/passive-style construction that is very common in Portuguese, especially on signs, notices, menus, and advertisements.

So:

• Aceita-se Pix = Pix is accepted / We accept Pix
• Não se aceita cheque = Checks are not accepted / We do not accept checks

This structure is often called the passive with se or synthetic passive in grammar explanations.

Because não normally pulls the clitic pronoun se before the verb.

So in standard Portuguese:

• aceita-se
• but não se aceita

This is a basic word-order rule with clitic pronouns. Negative words like não trigger proclisis, which means the pronoun goes before the verb.

So:

• Aceita-se Pix.
• Não se aceita cheque.

and não aceita-se sounds wrong in standard Portuguese.

Because each verb needs its own construction.

The sentence has two separate verbal units:

• Aceita-se Pix
• mas não se aceita cheque

The se belongs to aceita each time. You cannot usually let the first se carry over to the second verb.

That is why Portuguese repeats it:

• Aceita-se Pix, mas não se aceita cheque.

This makes the structure balanced and clear.

Because in this kind of short notice, Portuguese often uses the noun without an article when talking about payment methods or categories in a general way.

So these are very natural:

• Aceita-se Pix
• Não se aceita cheque
• Aceitamos cartão
• Não aceitamos dinheiro

This is especially common in signs and commercial language.

Adding an article would usually change the feel:

• o cheque can sound more specific or less sign-like
• um cheque would suggest a specific individual check

Without the article, the sentence sounds broad and generic: it is talking about the payment method as a category.

Portuguese often uses the singular to talk about something in a generic category.

So não se aceita cheque means that check payments are not accepted in general. It does not mean only one single check.

This is very common in signs and business language.

You could also find plural wording in other contexts, such as:

• Não se aceitam cheques

That is also possible. But in short commercial notices, the singular generic form is very natural.

Yes, absolutely.

That version is very common and often sounds a bit more direct and conversational:

• Aceitamos Pix, mas não aceitamos cheque.

The difference is mostly style:

• Aceita-se Pix, mas não se aceita cheque
  = more like a written notice, sign, or formal announcement
• Aceitamos Pix, mas não aceitamos cheque
  = more direct, with an implied we

Both are natural in Brazilian Portuguese.

In standard grammar, yes, usually.

For example:

• Aceita-se cheque.
• Aceitam-se cheques.

And:

• Vende-se casa.
• Vendem-se casas.

That is because, in the traditional analysis of this structure, the noun acts like the grammatical subject of a passive sentence.

However, in everyday Brazilian Portuguese, many people use the singular verb even with a plural noun, especially in speech:

• Aceita-se cartões

You may hear that, but in formal writing and in grammar-based teaching, aceitam-se cartões is the expected form.

Because Pix is a proper name: it is the official name of Brazil’s instant payment system.

So it is written with a capital P:

• Pix

It is treated more like a branded or official system name than like a common noun.

By contrast, cheque is a regular common noun, so it is not capitalized.

Both can express a passive idea, but aceita-se is much more natural in short notices.

Compare:

• Aceita-se Pix.
• Pix é aceito.

The first sounds like a normal sign or business notice. The second is understandable, but it sounds less idiomatic for this kind of message.

Portuguese often prefers the se-construction for short public statements like:

• Aluga-se apartamento.
• Vende-se carro.
• Precisa-se de funcionário.
• Aceita-se Pix.

So this pattern is worth learning as a very common real-world structure.

It is very natural in written public language: signs, store windows, menus, ads, and notices.

So you might really see:

• Aceita-se Pix
• Não se aceita cheque
• Aceita-se cartão
• Não se aceita dinheiro em espécie

In everyday speech, though, many Brazilians would more likely say:

• A gente aceita Pix, mas não aceita cheque
• Aceitamos Pix, mas não aceitamos cheque

So the sentence is real and natural, but it has a slightly notice-like or posted-information feel.