En ese banco cobran una comisión si haces la transferencia en ventanilla.

In Spanish, it’s very common to use the 3rd person plural (cobran, dicen, hacen) with an unspecified subject to mean “they / people / the staff”. Here it effectively means the bank (staff) charges a fee, without explicitly repeating el banco as the grammatical subject.

Cobrar = to charge / to collect (money) (from someone).
Pagar = to pay (money to someone).
So cobran una comisión means they charge a fee (you are the one who ends up paying it).

Una comisión introduces it as “a fee” in a general sense (one of the bank’s fees). If you were talking about a specific, already-known fee, you might see la comisión. In many real contexts, both can appear, but una is very natural for stating a policy: “they charge a fee if…”.

In Spain, comisión commonly means a bank fee/charge, e.g. a fee for a transfer, maintenance, withdrawing cash, etc. It’s not a “commission” in the salesperson sense here; it’s a service fee.

In Spanish you often “do” an operation: hacer una transferencia = to make a bank transfer. Other common alternatives are realizar una transferencia (more formal) or efectuar una transferencia (quite formal/banking style).

Spanish uses the present tense after si to talk about real/likely conditions in the present or future:

• Si haces la transferencia en ventanilla, cobran… = If you make the transfer at the counter, they charge…
  You don’t use a future tense after si in this type of sentence.

Here si clearly means if because it sets a condition: they charge a fee if you do it at the counter. Si can also mean whether (indirect questions), e.g. No sé si viene = I don’t know whether he’s coming. The structure tells you which one it is.

En ventanilla means at the counter / at the teller window, i.e. you go inside the bank and do it with staff. In Spain this is a very common phrase for in-person banking, contrasted with por internet / online, por la app, or en el cajero (ATM).

Spanish commonly uses en to express the “place/method” where a service is done:

• en ventanilla = at the counter
• en línea / en internet = online
  So en here is more like “at/by means of”.

Ese points to something not right next to the speaker, often something already mentioned or “that one (over there/that one you mean)”.

• este banco = “this bank” (near me / the one I’m pointing to as ‘this’)
• ese banco = “that bank” (the one you’re talking about / over there)

Because it’s describing location (where they charge): in/at that bank = en ese banco.
A ese banco would usually be used for movement/direction: voy a ese banco = I’m going to that bank.

Yes. A very common alternative is the impersonal/passive-like form:

• En ese banco se cobra una comisión si… = A fee is charged at that bank if…
  Both are natural; cobran sounds slightly more “people/staff do it,” while se cobra sounds more like a policy statement.

Yes, haces is tú. In a formal notice aimed at customers, you might see:

• si hace la transferencia (usted)
  Or a more impersonal style:
• si se realiza la transferencia en ventanilla (very formal).