You're doing all the work for your customers - you don't want to let the invoicing and billing to slip.  And what happens when someone's late?  How are you following up?  What's your billing schedule like?  Are you getting paid???

Entrust a virtual assistant to take care of billing for you.  It gives your customers a point of contact when they have billing questions or problems.  It gives you someone to do all the scheduling and ensure that the income is coming in on time.