Not be negative about bestfreevoice, or any other invoicing tool, but it always seems like developers and early-stage entrepreneurs don't understand invoicing.

First major disconnect is that not every country uses invoices, but may use receipts instead. This is true for the USA for example, so many US devs (for example: Stripe in the early days) are not familiar with the concept of invoicing. Technically there is no difference between receipts and invoices, so if you're not familiar with the concept of invoicing, just read this post with /s/invoice/receipt in mind.

The point about invoicing is to act as a non-mutable entry into the ledgers of both parties (seller and buyer). In most countries (especially EU) invoices are mandated by law for B2B transactions, and so is keeping accounts (aka bookkeeping). So for invoicing to be practical it needs to be tied to your books/accounts. Because of this, any business will use some bookkeeping/accounting software, which will have invoicing capabilities built-in. Invoicing as a standalone product doesn't make sense if you have to import it all into your ledgers later.

Then there is the 'design' trap, which many invoicing startups seem to fall for. Invoices are weird things. They are basically very, very inefficient artefacts from the past. An invoice is just a very little amount of transaction data exchanged between buyer and seller. In the days of physical bookkeeping (actual paper books) paper invoices made sense, but nowadays it is all done digitally. So the invoice is effectively a machine-2-machine interface, but for all sorts of legacy reasons we still wrap them in PDF with a fancy design that looks great for humans, but it effectively impossible for machines to read.

There are all sorts of attempts made to improve upon this situation (like OCR, and nowadays AI to extract data from PDF invoices). There are open structured data formats such as UBL to replace / augment PDF invoices, but due to all sorts of politics and lobbying the open standards have been doomed from the beginning. There is a lot of money made in accounting software, and they all rely on vendor lock-in. The major accounting software vendors have very strong incentives to keep us from adopting UBL et al, and most of the established accounting product suck, but you can't easily migrate so you'll be stuck with it.

If you run or own a business, treat your books as an asset of your business, a very important asset for that matter. Books are kept in accounting software, which is typically part of a larger software suite which also features tax filing, HRM, asset management, invoicing, etc. In fancy business terms this is often called ERP. But think of ERP as just your central database, or your 'books'.

Choosing your accounting software an important decision. Choose accounting software that allows exporting your data (very important!), that has an API (also very important), and preferably a web interface. It should be always available, so on-premise software is out.
For entrepreneurs: choose your own accounting software, do not be tempted to hire an external bookkeeper that keeps the books in 'their' systems (accountant lock-in). Don't let an accountant recommend your software either, they get huge kickbacks from the software vendors (vendor lock-in).
Every sale—whether this being PoS, invoicing, or a payment integration like Stripe—should automatically registered as a ledger entry in the books, preferably with an invoice document attached. Here you can see why an accountant who keeps your books in their systems won't work, you don't want to be stuck having to periodically send an email (or shoebox) filled with invoices for them to process. Your books should be owned by the business, should be automated (at least for the receivable side), and always be up-to-date. You can then give an(y) accountant access to your books for them to do audits, tax filings, etc. For a business, the books are the central database of the business, everything else revolves around it. Do not be tempted to write your own, instead integrate with existing solutions while avoiding vendor lock-in as much as possible.

Integrating your business with the accounting software is an ever-ongoing part of your software development efforts, so not underestimate it. Accepting payments is hard, making sure it is well registered in your books is equally hard. It takes _much_ more time than you'd think (most first-time entrepreneurs actually don't consider it at all). There are no silver bullets here.