Hello, LaGraves, and congratulations on launching your business. There are a few invoicing programs out there, the most popular I believe is Quickbooks and Freshbooks. I provide invoicing to only one client, which is a very small-scale deal compared to what I am sure other skilled "invoicers" provide. And my particular client only accepts company checks, which makes my invoicing job extremely simple.
For my own business, I am only at a minimal invoicing level and have solely been using PayPal for my invoicing needs.
I noted that you mention that you are going to provide invoicing as a service. Have you done this before?
The great thing about being a VA is that you can always expand your learning. There is always so much to learn! Take the time before launching to take a bookkeeping class, attend tutorials in Quickbooks, etc. Become as skilled as you possibly can so that when a client asks for help with invoicing, you will be more than capable of assisting with their needs. But until you have that skill down pat, I would not offer it as a service unless you let your client know up front that you are new at it.
With that said, we all have to learn somewhere. And sometimes the only way of learning is to do it. Just be honest with your clients. Ahhh, I rambled. And I'm not even an invoicing expert.