Hi Cole,
This is a quick A/R invoice example using the Sage100.ObjectManagement nuget package. www.nuget.org/.../Sage100.ObjectManagement

using Sage100.ObjectManagement; /* https://www.nuget.org/packages/Sage100.ObjectManagement */
using Sage100.ObjectManagement.Interfaces;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net;


namespace SageCityDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            var homePath = @"C:\Sage\Sage 100 Advanced\MAS90\Home";
            var company = "ABC";
            var user = "boiUser";
            var password = "****";
            var module = "A/R";
            var taskName = "AR_Invoice_ui";
            var busObjName = "AR_Invoice_bus";
            var regularInvoice = "IN";

            var sessionLogon = new SessionLogon
            {
                CompanyCode = company,
                UserLogon = new NetworkCredential { UserName = user, Password = password },
                ModuleCode = ModuleNames.AccountsReceivable,
                ModuleDate = DateTime.Today
            };

            var itemCodeList = new List<string> { "/WIDGET", "/100-AA" };

            using (var sySession = new MasSession(homePath))
            {
                // Create session object.
                sySession.InitSession(sessionLogon);

                // Create business object.
                using (var arInvoice = sySession.CreateObject<ILineEntry>(busObjName, taskName, module))
                {
                    // Create new batch if batches are enabled.
                    if (arInvoice.BatchEnabled)
                    {
                        arInvoice.SelectBatch();
                    }

                    // The dynamic PvxObject pointer is available to access pvx methods directly.
                    var invoiceNo = string.Empty;
                    arInvoice.PvxObject.nGetNextInvoiceNo(ref invoiceNo);

                    // Start a new invoice.
                    arInvoice.SetKeyValue("InvoiceNo$", invoiceNo);
                    arInvoice.SetKeyValue("InvoiceType$", regularInvoice);

                    if (arInvoice.SetKey() != Mas.edtNew) throw new ApplicationException($"Failed to create a new invoice. {arInvoice.LastErrorMessage}");

                    // Set the customer for the invoice.
                    arInvoice.SetValue("ARDivisionNo$", "01");
                    arInvoice.SetValue("CustomerNo$", "ABF");

                    // Create line detail.
                    foreach (var itemCode in itemCodeList)
                    {
                        arInvoice.AddLine();
                        arInvoice.Lines.SetValue("ItemCode$", itemCode);
                        arInvoice.Lines.SetValue("QuantityOrdered", 1);

                        if (!arInvoice.Lines.Write()) throw new ApplicationException($"Failed to write the line.  {arInvoice.Lines.LastErrorMessage}");
                    }

                    // Save the new invoice.
                    if (!arInvoice.Write()) throw new ApplicationException($"Failed to write the invoice.  {arInvoice.LastErrorMessage}");

                    Trace.WriteLine($"New invoice {invoiceNo} was created.");
                }
            }
        }
    }
}