Check self service to see if the overpayment is in Available Funds. You can just leave it there and it will go towards the next renewal.
I can understand your frustration with this. Since Public Mobile is prepaid, you will only be charged ahead of time for your plan minus any rewards you have. Now you can add to your balance to have let us say $50 in it. You will only be charged for your plan amount.
If the amount of available fund in your account is the same as the over payment, PM left your over payment on your account. The available fund will be used for your next renewal. If you ask for refund, it may not arrive in time to pay for your next renewal.
If your over payment is not in your account, create a moderator support ticket for help using the ? button at the bottom right corner of this page.