Learn Hardcore Indonesian: Saya membayar biaya langganan lewat transfer.

Both are correct. bold Membayar is slightly more formal/complete; bold bayar is very common in everyday speech. So you can say bold Saya membayar biaya langganan lewat transfer (neutral/formal) or bold Aku bayar langganan lewat transfer (casual).

Is bold biaya langganan the standard way to say subscription fee?

Yes. bold Biaya langganan literally means subscription fee and is widely used. You’ll also see bold biaya berlangganan in formal contexts; it means the same thing. In casual speech you can shorten to bold bayar langganan (pay the subscription).

What’s the difference between bold biaya, bold harga, bold tagihan, and bold iuran?

bold biaya: fee/charge/cost (administrative fee, subscription fee, shipping cost).
bold harga: price (the listed price of a product or service).
bold tagihan: bill/invoice (the statement you receive to be paid).
bold iuran: dues/levy (regular contributions, often for communities or associations).

What else can bold langganan mean besides subscription?

bold Langganan can also mean regular/favorite, as in bold warung langganan (a place you regularly go). Related words: bold pelanggan (customer/subscriber) and bold berlangganan (to subscribe), e.g., bold Saya berlangganan koran itu.

Is bold lewat the best preposition here? How does it compare to bold melalui, bold via, bold dengan, bold pakai, bold menggunakan?

All are possible, with register differences:

bold lewat: very natural, everyday (by/through).
bold melalui: more formal.
bold via: borrowed and common in casual writing.
bold dengan/pakai/menggunakan: mean with/using; acceptable, but you’ll most often hear bold lewat/melalui with payment methods. Examples: bold bayar lewat transfer, bayar melalui transfer bank, bayar via transfer.

Should I add bold bank after bold transfer (bold transfer bank)?

Not required. In Indonesian, bold transfer by default implies a bank transfer. Add bold bank if you want to be explicit or distinguish from other channels: bold lewat transfer bank, lewat transfer BCA, lewat transfer ke e-wallet.

How do I show past or future time with this sentence?

Indonesian doesn’t inflect for tense; add time/aspect words:

Past/already: bold Saya sudah membayar biaya langganan lewat transfer; Tadi/Kemarin saya membayar…
Just now: bold Saya baru saja membayar…
Future: bold Saya akan membayar…; Nanti saya bayar… (casual).

Is the word order natural? Where should bold lewat transfer go?

Yes, bold Saya membayar biaya langganan lewat transfer is natural. The method phrase typically comes after the object. Fronting is possible for emphasis: bold Biaya langganan sudah saya bayar lewat transfer (more formal/emphatic).

Is bold Saya the right pronoun here? What about bold Aku or bold Gue?

bold Saya is neutral/polite and works almost everywhere. bold Aku is casual/intimate with friends or family. bold Gue/Gua is Jakarta slang/informal. Choose based on your relationship and setting.

Can I drop the subject and just say bold Membayar biaya langganan lewat transfer?

As a standalone statement, it sounds incomplete. Dropping the subject is fine if context is clear (e.g., in a checklist or a chat reply): bold Sudah bayar biaya langganan lewat transfer.

What’s the difference between bold membayar, bold membayarkan, and bold membayari?

bold membayar: to pay (neutral): bold Saya membayar biaya langganan.
bold membayarkan: to pay something on someone’s behalf: bold Saya membayarkan biaya langganan adik saya / untuk adik saya.
bold membayari: to pay for someone (beneficiary-focused, often repetitive/habitual): bold Dia sering membayari saya makan; Dia membayari adik saya biaya langganan.

Does bold lewat ever mean late or past? Could that confuse things?

Yes. bold Lewat can mean past/late in other contexts: bold jam tujuh lewat lima menit (7:05), bold kamu lewat batas waktu (you’re past the deadline). In your sentence it clearly means by/through, so there’s no ambiguity.

How would I make the sentence more formal or more casual?

Formal: bold Saya melakukan pembayaran biaya langganan melalui transfer bank.
Neutral: bold Saya membayar biaya langganan lewat transfer.
Casual/texting: bold Aku bayar langganan via transfer / Gue bayar langganan via tf.

Can I use bold -nya to mean the subscription fee we already talked about?

Yes. bold biaya langganannya can mean the specific/previously mentioned subscription fee (or his/her/its subscription fee, depending on context). Example: bold Saya sudah membayar biaya langganannya lewat transfer.