Adding Payments Total to Invoice (DAC Extension?)

Our company has an ongoing problem of customers paying twice for invoices that they pre-payed on because the invoice doesn’t show payments applied to that invoice. I have added a subreport to show an invoice’s payments. Now, I need a way to use the payment total on the main report, which I know can’t be passed from the subreport to the main report. I ask about this on StackOverflow before I knew this community existed and someone there suggested a DAC extension, which I’ve never done before. I’ve read through the 2019 R1 Customization guide and have a weak grasp on how DAC Extensions work. The problem is that all of the examples are shown for user entry fields and all of the examples I’ve found on StackOverflow are for user entry fields, too. I need a extension field that is calculated for the report and I haven’t found an example for that, yet. I’m waiting to get approval from the finance department to contact our third-party Acumatica support company, but in hope of getting it done as soon as possible, could someone here help me out? I’d really appreciate it. Thanks.