small AV installation company in need of invoicing

QUICKBOOKS

its the industry leader that most bookkeepers use and also easiest to learn and use for owner newbies. It has an on-line version that permits you to use it in the go. 

(I’m a CPA)

I do some consulting on the side.  For simple invoices I use a template in Excel.

@akg_ca +1, Quickbooks. Especially if you hope the business will do well, it can handle small to medium size businesses with no problem. And being online, it's always available wherever you are and is backed-up automatically.