BillingPlatform allows you to apply credits to invoices to account for customer service actions or incorrect billing. Please see the tutorials for more information on creating/allocating credts.
Payments - In order to apply your customers payments to their invoices, or have them apply their own invoices through the customer portal, you will want to follow the link for more information on how to configure these options.
Refunds - If you need to refund a customers payment, or a portion of their payment, you will need to apply a refund. Please follow the link for more information.