Would be much better if you clicked on Go Advanced when posting and use a table, or create in Excel and dump here to automatically create a table.
Wouldn't your InvoiceID be linked/related to your OrderID because it's not related to the Customer as you suggested?
If you've added InvoiceID field to an existing table, you don't append, you update. Only reason I can think of for adding that many records is that there's more than one table in the query and there's no actual join (a Cartesian query) - that and the possibility you're using the wrong query type. Not sure I get the idea of adding just any invoice to such a header. I guess that the detail records are not for invoice payments and maybe not related to orders either.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.