Good article AngryWarrior!
I know that topic is about conversion, but as a side note, I would add p.s. at the end:
As always, as we speak about WordPress sites, updates are made from time to time, it is important to have site backups and make those updates on staging sites in the first place, to see does all will work the same way. With any kind of issue on the site, and especially on the checkout page the conversion steps will be not valid anymore.
It is similar to a local store where you can see multiple people shopping and all of them end up at the checkout, but the cashier says that the electricity went out, the cash register is not working and she cannot handle anyone. This plays an important role. Not only will customers not buy, but there is a high risk that they won’t come back.