I switched my membership off only a few minutes after paying for it — where does that leave me?

Switch it off inside the introductory window and nothing is drawn from your balance. Switch it off after the window closes and the amount has already been taken, though Isochron Plus stays unlocked right to the last day of the term you paid for. After that day the membership will not renew on its own and the account settles back to the free toolset. Anything about the payment — a duplicate line, an expired card, a missing receipt — sits with the billing service that processed it, so raise it there. Where their refund window still covers the order, they can usually refund it.

When I take the twelve‑month term, is the whole sum drawn at once, or does it arrive as the monthly figure in the promo?

Terms are billed up front, so the full amount for the option you picked leaves your balance the moment the purchase is confirmed. Isochron Plus then stays open to you for the whole of that term, with nothing further to pay until it ends.