FROM PaymentAA

...
// define a new invoice 
           Emp_Invoice__c newinvoice = new Emp_Invoice__c();
           
           // update the value of the account it relates to to the same as the sales prospect
           newinvoice.Account__c = spAcc;
           
           // relate to sales prospect
           newinvoice.Sales_Prospect__c = spID;
           
           // sets the date on the invoice
           newinvoice.Invoice_Date__c = date.today();
                      
           // commits the new invoice to database
           insert newinvoice;

           // returns a list of all the order items associated with sales prospect
           List<Order_Item__c> orderitems = [select Name, Order_Line_Total__c from Order_Item__c where Sales_Prospect__c = :spID];
           List<Invoice_Item__c> ii = new List<Invoice_Item__c>();
           // iterate over all the line items in the order
           for (Order_Item__c oi : orderitems) {
               // create a new invoice item corresponding to each order item with the same values
               ii.add(new Invoice_Item__c(Invoice__c = newinvoice.Id, Order_Item__c = oi.Id, Line_Total_ex_GST__c = oi.Order_Line_Total__c));
           }
           insert ii;
...



AND THE WHOLE Class.InvoiceAction This Triggers from "Invoice_Item__c" after insert / after update 

public class InvoiceAction {
    
    public static void updateInvoiceAmount(Invoice_Item__c[] newInvItems) {
        //array of invoices that have a new amount paid
        List<Emp_Invoice__c> myInvoices = new List<Emp_Invoice__c>();
        //iterate over all the new invoice items in this batch
        for (Invoice_Item__c thisInvItem:newInvItems ) {
            //ingnore invoice items with out an invoice 
            if (thisInvItem.Invoice__c != null){
                //get all invoice items for this invoice
                List<Invoice_Item__c> allInvItems = [SELECT Id, Line_Total_ex_GST__c, GST_Amount__c, Amount_Paid__c FROM Invoice_Item__c WHERE Invoice__c = :thisInvItem.Invoice__c limit 1000];
                // get the invoice for this invoice item and lock it
                Emp_Invoice__c thisInvoice = [SELECT Id, Amount_ex_GST__c, GST_Amount__c, Amount_Paid__c FROM Emp_Invoice__c WHERE Id = :thisInvItem.Invoice__c limit 1 for update];
                decimal sumApPaid = 0.0;
                decimal sumLtPaid = 0.0;
                decimal sumGstPaid = 0.0;
                // sum up all the invoice items for this invoice 
                for (Invoice_Item__c cInvItem:allInvItems ) {
                    if(cInvItem.Amount_Paid__c != null){
                        sumApPaid += cInvItem.Amount_Paid__c;
                    }
                    if(cInvItem.Line_Total_ex_GST__c != null){
                        sumLtPaid += cInvItem.Line_Total_ex_GST__c;
                    }
                    if(cInvItem.GST_Amount__c != null){
                        sumGstPaid += cInvItem.GST_Amount__c;
                    }
                }
                // update the amuount paid for this invoice
                thisInvoice.Amount_Paid__c = sumApPaid;
                thisInvoice.Amount_ex_GST__c = sumLtPaid;
                thisInvoice.GST_Amount__c = sumGstPaid;
                
                // add this invoice to the array of invoices that have a new amount paid
                System.debug('[InvoiceAction -> executeTriggerAfter] adding amount paid=' + thisInvoice.Amount_Paid__c + ' to invoice id=' +thisInvoice.id);
                myInvoices.add(thisInvoice);
            }
        }
        //update all invoices that have a new amount paid
        update myInvoices;
    }
    

}