Could you please not do this making-it-personal trope on HN? It's internet incivility that mars your otherwise fine comment.
WordPress (with WooCommerce) is actually the most popular e-commerce platform in the world and powers around 40% of all online stores.
https://trends.builtwith.com/shop (Click "The Entire Internet" tab)
If you install WooCommerce you'll see that it's a full-blown e-commerce that can do receipts, track orders, and just about everything else.
I recently used it for a client website, developing all the stuff that they got for free would've cost 20,000.
I don't love WordPress, but it's extremely convenient if you don't like to waste time rewriting stuff that is already available as free, debugged software.
I built this because people were asking me for it before I sold my previous software business. I know tools like WaveApps (and countless others!) are really useful but I think invoicing built into WordPress does meet a need.
I think they also have a paid support option but I can't remember, it's been a while since I used them.
The simple answer to why I use Codecanyon is access to a large, targeted customer base that is ready to spend money. That's not an easy thing to replicate. No matter the combination of PPC, content marketing, influencers and direct contact I try.
Believe me I've tried on many occassions over the last few years to setup direct sales of my various plugins and software. But I can't seem to drive enough targeted traffic to match the income via marketplaces like Codecanyon.
1) Editing paid invoices shouldn't be allowed after a payment has been made.
2) Quotes should have an accept or sign option, the quote list should reflect the status.
3) Quotes should also have templates similar to invoices
4) Dashboard widget reflecting outstanding payments, recent payment stats, etc. Could be handy.
I used to have other plugins that were sold directly, plus a couple in development which I may sell directly as well. When selling directly I have full control over the updates but through Codecanyon I do not have that benefit.
My preferred option would be to add support forever (helping people with the product, questions and so on) but actually limiting updates for a certain period, perhaps 1 year. That way I am paid for continually developing and improving the plugin but customer support is included always, which I think is important as a small business.