Learn Hardcore Russian: Я сделаю платёж онлайн прямо сейчас, чтобы потом спокойно отдохнуть. - I will make the payment online right now so that I can rest calmly later.

Сделаю is the perfective future of делать/сделать. Perfective is used for a single completed result: I will make the payment (and it will be done).
Делаю is imperfective present (I am doing / I do). For “I’m paying right now” you could say я делаю платёж прямо сейчас, but it emphasizes the process, not the “done” result.

What does the prefix с- in сделать add?

In сделать, the prefix helps form a perfective verb meaning “to do/make and finish.” It often implies completion: do it and have it done.

Why is it платёж, not оплата? Are they interchangeable?

They overlap but aren’t identical:

платёж = a payment as an individual transaction (a конкретный платёж: one payment).
оплата = the act/process of paying or payment/settlement in a more general sense (payment for something).
In this sentence, платёж sounds like “a single payment transaction online.” Я сделаю оплату онлайн is possible, but can sound a bit more formal/“settlement-style,” depending on context.