Do billable expenses have to be tracked through an income account? Can they be linked to an expense account and be deducted from the P&L?

@accounting-maine 

Sure, they can be linked to an expense account.  Just set up a product/service linked to the expense account you want to reduce.  Then, when you add the billable expense to the invoice, select that product.  Billable expenses are nothing more than a reminder to bill your customer for the cost marked "billable" on a bill.  The account linked to the product/service is the offsetting credit to the A/R debit on the invoice.