That can easily be done from the app.
At the bottom should be shown Sim Subscription
Select Turn Auto-Renew On and there are 3 possible methods of payment:
1. PayPal
2. Credit Card
3. Wire Transfer
There should be a 0 charge on your payment method for a payment confirmation, afterwards the system will automatically take the full payment so don't be alarmed if it displays 0.
NOTE: It is possible to do a one-time payment, all you need to do is activate the auto-renew allow the payment to be charged, then go back to the same Sim subscription page, and choose Cancel Auto-Renew. That's the only way to do a one-time payment for now.