If any of your Revenue Items are configured to have a Related Revenue Item assigned to it, you will now be prompted to modify the invoice when applying a payment to that type of line item. This includes voluntary contributions that add an additional line item to invoices, or it can include "rebalance" options that split the original line item down into multiple items.
To modify an invoice, simply apply a payment to that invoice through any method* (click here for details on entering a payment). You'll see a message prompting you to modify the invoice, because one of the line items is configured to have Related Revenue Items:
Click Yes, Modify Invoice. The related line items will be displayed, with the default values filled in based on your settings to add them to the invoice, and the corresponding amounts. You can click in the Custom Amount field to change the amounts if necessary, or uncheck the "Opt In" checkbox to remove one of the related items completely.
The new/proposed invoice will be displayed at the bottom based on your selections, and the summary values at the top will update as well.
Once you've entered the appropriate modifications, click Save. The invoice will be updated, and you can continue to apply the payment to the modified invoice as usual.
* Note: the bulk payment method using "Generate Payments for Selected Invoices" in the action menu on the invoice selector screen is the exception. The bulk payment tool does not make related revenue item modifications as it is specifically designed for applying payments to multiple invoices at once.