What Joseph and John said!
http://www.databasecreations.com has a commercial package which includes
some sales invoicing functions, among many others, that comes with source
code. It is called Business Professional Edition, and used to have the
intriguing name "Yes, I Can Run My Business With Access". I've never used
it, so can't recommend for or against it. There are some other Access-based
accounting packages described at MVP Tony Toews' site,
http://www.granite.ab.ca/accsmstr.htm.
But, if you were expecting to get a free package that just fits your
requirements, you might have better luck looking for freeware or open source
applications that include configuration options. They wouldn't be Access,
but they might fit your actual needs just as well or better than a
commercial package written in Access.
Larry Linson
Microsoft Access MVP