I am trying to create an automation in a Jira Work Management Project that will add the value from the custom field in a task called "Invoiced Amount" which is "customfield_10156" and add to the custom field in the Epic called "Total Payments Received for FY" which is "customfield_10158" when the Task is transitioned from "Invoiced" (In Progress) to "Payment Received" (Done).
I was able to get the basic automation done but have had zero luck in getting it to sum the Invoiced Amount to the Total Payments received. My research tells me that you cannot sum custom fields like this via JQL and I am not familiar with Json and am basically stuck. I have done some research and believe that I need to send a Web Request, but could really use some help.
I am also having trouble getting it to select the issues in the Look Up without explicitly stating the parent ID. I figured that would be relatively easy once I figured out the summing part, but suggestions are welcome.
Also, I don't have a preference if it adds the invoiced amount of the card being transitioned to the Total or if it just takes all of the cards in the Invoiced Paid column related to the parent and sums them all up and overwrites the existing total.

Thanks in advance!
Shelley