You can use the iBatchCallback global interface to hook into PSA billing processes such as:
These interface hooks are called during PSA billing processes and each method is called even when a process is executed inline rather than as a batch. To use these hooks, you must write classes that implement the iBatchCallback interface. Once you have written a class to handle a billing process, you must instruct PSA to use it. To do this, add a value containing the name of your class to the appropriate billing Interface configuration option in the Billing configuration group. For information about configuring PSA to use a custom billing class, see:
For information about creating customSalesforce billing classes for PSA billing processes, see the PSA API Developer's Reference. For more information, see FinancialForce PSA Documentation in Community.