Ecommerce without shopping cart

I am looking for an ecommerce solution for an ebook. This is the only item sold on the website.

What’s the simplest way to set it up?

What do you need it to have? ie taxes, receipts, instant digital download, etc

Instant digital download. The download in a confirmation email would probably suffice.

Such as with a form plugin.

Look into Shareable online pay links and buy buttons | Square Online Checkout. Just add the link as a button to your site to enable checkout. Not sure how it would work with downloads though.