Invoicera is the best enterprise billing software.
Invoicera helps users to simplify and automate business processes. Its features include online invoicing, expense management, recurring/subscription, time tracking, multi-currency support, etc. Invoicera also helps users integrate with existing legacy systems and also helps users to customize the Invoicing process as per your business needs.

Features:

>Create Invoices from anywhere

>Easy account management

>Send multiple invoices in one go.

>Access at any time

>Three-layer security

>Track your expenses

>Customized templates

>Get your payments faster with multiple payment gateways.

>Ar & Ap management

>Automatic Payment Reminder

Speaking of that, can anyone recommend a company that works in the medical field and can provide decent billing services?

I think I have the best solution for you. Personally, I managed to find  https://fortismedicalbilling.com/asc/, and I highly recommend you check these guys out. It took me a while to discover them, but it was all worth it. I've been working with them for a few months, and I'm sure you won't be disappointed.