If you want something cheap: Excel or Word. Simple, flexible and if you’re not sending a lot of invoices, also the easiest. I’d do this as long as your bookkeeping is manual as well.
Once you start having more activities, sending more invoices have more expenses and you want to automate, I’d look for something that integrates expenses, invoicing and bookkeeping. One system that does it all is just such a relief but connected systems work as well
You forgot the most annoying threads: The ones complaining about the sub without adding any value