Single Product Stores – Best Practices

Hi everyone, I'm looking for examples of simplistic and really fast checkout flows.

Basically, I want to find some best practices that could be used for conversion rate optimization of a single item (cheap, impulsive product) store.

I believe this could be a valuable resource of tips & tricks / inspiration for many users on this subreddit. So, any recommendations would be very helpful to kick this off – thanks in advance!