Eu acabei de receber a fatura e vou pagar pelo aplicativo.

Acabei de + infinitive is a very common way to say you did something just now / recently.

• Eu acabei de receber... = I’ve just received...
  Grammatically, acabei is the verb acabar conjugated in the pretérito perfeito (simple past), and it’s followed by de
  • an infinitive (receber).

Yes. In Brazilian Portuguese, subject pronouns are often omitted because the verb ending already shows who the subject is.

• (Eu) acabei de receber a fatura... is natural either way.
  Including eu can add emphasis or clarity, especially in longer contexts.

Because the speaker is I:

• eu acabei = I just finished / I’ve just
• você/ele/ela acabou = you/he/she just
• nós acabamos = we just
  So acabei matches eu.

They overlap, but in Brazil they tend to be used like this:

• fatura: often an invoice or especially a credit card bill (fatura do cartão). It can also be a formal bill from a company.
• conta: a general bill (water, electricity, phone): conta de luz, conta de água.
• boleto: a very Brazilian payment method/document (boleto bancário) used to pay many bills (often with a barcode), either in a bank, lottery shop, or in an app.

Portuguese frequently uses definite articles where English might not. Here a fatura means the invoice/bill—a specific one the speaker just received.
Also, fatura is feminine, so it takes a (not o).

Vou pagar is the common “near future” formed with ir (to go) + infinitive:

• vou pagar = I’m going to pay / I’ll pay
  It’s extremely common in spoken Portuguese and often preferred over the simple future (pagarei), which can sound more formal.

Yes, but the nuance changes:

• vou pagar: neutral, everyday, conversational.
• pagarei: more formal, sometimes sounds like a promise/commitment or written style (customer service, contracts, official messages).

Pelo is a contraction of por + o:

• por o → pelo
  In this context, por often means via / through / using:
• pagar pelo aplicativo = pay through the app / using the app

Both can work, but they emphasize slightly different ideas:

• pagar pelo aplicativo: paying via/through the app (the app is the method/channel).
• pagar no aplicativo: paying in the app (focus on the location/interface).
  In everyday Brazilian Portuguese, pelo app/aplicativo is very common for “using the app.”

Both are common:

• aplicativo is the full Portuguese word (a bit more formal/neutral).
• app is extremely common in speech and writing.
  You’ll often hear: vou pagar pelo app.

• aplicativo: stress on TI → a-pli-ca-TI-vo (approx. ah-plee-kah-TEE-voo)
• fatura: stress on TU → fa-TU-ra (approx. fah-TOO-rah)
  (Exact vowel sounds vary by region, but the stress pattern is consistent.)