Invoicing is something I cover at length in my Expert Series, yes. Most of the Expert series covers all kinds of different aspects of invoicing, sales tax, vendors, customer discount rates, etc. So I'd recommend that, yes.

I am going to be also building an Invoicing system for my ABCD database which will incorporate what I started in the TechHelp videos and build on that. I'll probably put together an additional standalone Invoicing Seminar / Template to go with it as well. So much to do, so little time. But yes, if you don't want to wait - the Expert series has it ready NOW.

And if you want Inventory Management, check out Developer 23.