Welcome to the AGL Community!
You are free to make miscellaneous payments to your account as you fit using any of the payment methods listed on the second page of your bill. These payments will simply accrue on your account as a credit, and will then apply to your next bill!
If you have set up a direct debit on your account then this amount will be withdrawn automatically from your bank account. You can check the status of your direct debit by logging into your AGL My Account here.