It seems to me that since the invoice number is already in Access, that you would, or at least should in my opinion, use Access to generate your invoices. The sample Northwind.mdb has an invoice in it as well as orders etc.
Having developed a number of applications in Access, I think this would be your best solution. I use Excel only for spreadsheet type tasks/analysis. It wasn’t intended to be an invoice generator, so although you might be able to get it to work, I think Access is your best bet.
A few good sites for Access assistance/information tips & tricks are:
http://www.mvps.org/access/ Access forum
http://www.jmhare.com/wildhare2.htm Jim Hare’s access site
http://www.forumco.com/jmhare/ Jim Hare’s access forum
http://www.tek-tips.com/index.cfm Tech forums
http://p2p.wrox.com Tech forums
Hope this helps you out.
(no relation to Jim Hare though I wish I had some of his knowledge)