Invoices

[BGP]

I probably don't understand how to create an invoice properly 
but....here's what I've done-

I create an Invoice that includes the hours I've worked and the amount 
per hour.  That works fine.

Then I include the material charges for the work done.  The material 
charges are entered in the description area.  But, I can't include 
Expense accounts in the invoice.  So, I put the a MINUS sign next to the 
material charges and it all works out well.

Is that the way to do it or is there another way?