Just some random feedback, all of course my personal opinion:

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.

Thanks for taking the time to give feedback like this. 1-3 definitely agree and I've just put them on the backlog. I've already got 4 on there and will likely add it in the next update.

Site mentions "Free Lifetime Updates" but Codecanyon sales page says "6 months support from QuipCode" with a paid option to extend to 12 months. That seems somehow misleading and may potentially damage trust and cost customers.

It's misleading from the Codecanyon side unfortunately. You do get all updates forever on items bought through Codecanyon regardless of if your support has run out. The support part refers to answering questions and the like and it's defined by Codecanyon.

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.