Hi everyone;
I am almost done with a prototype for an invoice.cls.  This is intended to include a basically declarative format for invoices without the manual "generate a layout" approach we have now.  It would then allow for better handling of layout for letterheads (using a .sty instead), and an ability to override formatting of the invoice boilerplate and footer.
I expect to post code here along with an example to test its use sometime in the next week or two.
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Best Wishes,
Chris Travers
Efficito:  Hosted Accounting and ERP.  Robust and Flexible.  No vendor lock-in.