Learn Portuguese (Portugal): Podes cancelar a subscrição contanto que pagues a penalização. - You can cancel the subscription provided that you pay the penalty.

The phrase contanto que (provided that) is a conditional conjunction. Because it introduces a condition that must be met for the first action to happen, it always triggers the subjunctive mood.

Why is there a "u" added to the spelling of pagues?

The verb is pagar. When conjugating it in the present subjunctive, the endings start with the letter "e" for AR verbs. To keep the hard "g" sound from the infinitive, Portuguese spelling rules require changing the "g" to "gu" before an "e". If it were written without the "u", the "g" would take on a soft "zh" sound.

Could I use desde que instead of contanto que here?

Yes, absolutely. Desde que is a very common alternative that also means "as long as" or "provided that". Just like contanto que, it requires the subjunctive mood, so the rest of the sentence would stay exactly the same.

Are subscrição and assinatura interchangeable for "subscription" in Portuguese?

In most everyday contexts, yes. Subscrição is frequently used for digital services, apps, and memberships. Assinatura is also widely used for streaming services, internet plans, and traditional magazines. Both are perfectly natural in this sentence.