You can log into your account and click "Make a payment". Then you can pay with credit card or voucher.
Or if you have auto-pay set up with a card you can call 611 to make a payment with your card. You can also pay with a voucher.
Link below is list of place you can buy a voucher.
Pay my bill
You have 2 choices:
1. Setup autopay with your credit card or debit card. You will get $2 off every 30 days with autopay enabled.
2. Purchase PM topup voucher