StickerBrad,
Since you are inserting already paid items, You'll want to look at using Sales Receipts rather than Sales Orders. You'll first create the Sales Receipt header (Table = SalesReceipts), and then insert the items purchased as Sales Receipt Lines (Table = SalesReceiptLines) The total from the Sales Receipt Lines will add up to create the total amount for the Sales Receipt.
I've attached a code snippet of how to accomplish this.
var MAPIsvc = new MethodAPIService.Service();
string account = txtAccountName.Text;
string email = txtLoginEmail.Text;
string pwd = txtLoginPassword.Text;
string recordId = "";
string tablename = "SalesReceipt";
string[ fields = "Customer,BillAddressAddr1,DueDate,TxnDate".Split(',');
string[ values = "Angy Trees,123 Fake St,2015-01-01 12:00:00,2015-01-01 12:00:00".Split(',');
txtResponse.Text = MAPIsvc.MethodAPIInsert(account, email, pwd, tablename, fields, values, ref recordId);
tablename = "SalesReceiptLine";
fields = "SalesReceiptRecordID,Item,Quantity,Rate,SalesTaxCode".Split(',');
values = (recordId+",Hours,10,10,NON").Split(',');
txtResponse.Text = MAPIsvc.MethodAPIInsert(account, email, pwd, tablename, fields, values, ref recordId);