You can allow your members to make payments for outstanding charges themselves through their Member's Portal.
To enable members to make payments online navigate to Settings > Members Portal > Billing > Enable Pay Outstanding Balance button.
A common case for this is when a member fails a billing payment. You can use the Task system to notify the member of the failure, and provide them with a link to their Member's Portal to re-attempt the payment or update their billing details.
For more information on using the Task system, see the article Setting up a new Task Type
Another case is where members wish to make a deposit to their account, which can be used for later charges or purchases.
To enable online deposits navigate to Settings > Members Portal > Billing >Enable Make Deposit button