The easiest way is to use your credit or debit card. However you still have the option to pay with cash upon delivery, Zain Cash andZain vouchers on registration. Keep in mind that you can use vouchers for first payment only if you already are in possession of an oodi SIM card. For day to day top-ups you can use Zain Cash and vouchers, besides your credit or debit card.
Please remember, your credit card issuing bank may charge additional fees for your transaction.