Available Funds in Self-Serve account are always used first. You can even have AutoPay activated (for AutoPay Reward and AutoPay bonus data) but never get your credit card charged, as long as you always have enough prepaid funds in Self-Serve to cover your next Plan billing cycle.
I used to deposit payment through PM payment vouchers every month. Now I just throw a hundred bucks at it every six months or so (I currently have just a cheap $15 Plan which is even cheaper after Rewards).
You can login to Self-Serve every now and then - once every few months? - to monitor your balance, if you like.
You get automated SMS notifications when payment date is approaching, when payment has been accepted, when payment has failed. Useful reminders, time to login Self-Serve and check the account balance.
If you don't have enough prepaid funds to renew your Plan then it will charge the remaining amount through AutoPay. But if you don't have AutoPay enabled then your Plan and phone service get Suspended (stop working) until enough funds are deposited to fully prepay your next billing cycle. They can remain Suspended for up to 90 days, then they get permanently Terminated (account shut down, everything attached to account deleted, SIM card irreversibly deactivated, phone number lost).
If your account is Suspended then just deposit payment and it will Reactivate, you start a new billing cycle, at Public Mobile you aren't even charged for the Suspended time, you are not penalized or fined in any way for Suspension and Reactivation (aside from being unable to collect Rewards for days of inactive service).
Don't go too wild, though. PM is all strictly prepaid but also has a policy of no refunds. You can't easily reclaim your money, you won't receive any pro-rated credits or discounts for unused portions.
