I have a problem here in New Zealand where apparently using the Order number for both Order and Invoice number causes problems in business accountancy etc... (i'm not an accountant).
Normally, when placing an order with a company, it has an order number. However, when you are invoiced for that order, the Invoice number is issued, especially if several orders are under one invoice. In Ubercart there is only an Order number and I assume this is to be used for both the Order and Invoice number.
So, I'm trying to figure out a was to add an invoice number to orders and I've come up with an idea that may work. In the database table uc_payment_receipts there is an auto-increment field receipt_id. I am thinking perhaps that could be used as the Invoice Number, then, this number should be called from the database and displayed on all reciepts, emailed reciepts and invoice templates etc.
BTW: In New Zealand, we also usually display the business GST number on invoices so, not necessarily have a GST or TAX field, but perhaps a 'misc' field that can display addittional info on invoices may be handy, otherwsie this has to be hardcoded in the templates.