Hi! Did you try to set up auto payment? You will see how much you owe after all your rewards? Do you see correct amount? If not, it is probably technical issue. And unfortunately, you have to wait until it will be fixed.
The most cheapest plan is $15 per month. Public Mobile does not have pay as you go plans. This plan works 30 days as long as you pay for that. Too look more plans simply follow the link https://www.publicmobile.ca/en/on/plans#plans-start