Ok, in Calc, if I change the output from western to utf-8, check to put all text fields in quotes, remove check for presenting data in same appearance, and then save, it will be read just fine by your import feature. Invoices import as credit notes to begin with until I go into each invoice and add the item from the item list. When I do that, the invoice balances out.
I think this is how I will do my integration for now. I add a quantity column. The only time invoices won’t import, is if they are missing required information. The fields in the importer mapping screen don’t tell me which fields are required however. Could you provide that list? When I have entries that complete that list, the import goes just fine.