Invoicing by itself is relatively simple. It's the payments and accounting interface that are the hard parts. Most business apps nowadays can't exist in isolation.
Ideally, an invoice can be emailed to the client with a link to make a payment. Once the payment is made, the invoice needs to be marked as paid but only after an appropriate accounting entry has been recorded. The idea is to make getting paid as easy and seamless as possible.
Hey
What country are you from?
What important accounting entries are you speaking of? And what accounting software do you use if any?
I might have a simple solution for you
USA. Accounts receivable/general ledger. Quickbooks Online is what I use personally. I work with clients who have similar needs using other accounting apps.
The point being that invoicing needs to offer multiple interface options --- or build invoicing directly into the accounting system which is what QuickBooks has done.
Ideally, an invoice can be emailed to the client with a link to make a payment. Once the payment is made, the invoice needs to be marked as paid but only after an appropriate accounting entry has been recorded. The idea is to make getting paid as easy and seamless as possible.
Good luck.