[ehnto]

It's too broad of a question to be answered generally I think. Each vendor has different goals and product lines that perform differently in this regard, and not all products can be bought in one click anyway.

For a shoe company, they realize that most people buy one pair at a time, so while they hope they can upsell you some shoe-shine they realise that their possibility to expand the cart is pretty low. I imagine one-click could work well for them.

A RC parts store on the other hand, there is huge room for cart expansion there. If you just sell someone an RC plane in one click, they may not even be able to use it without more purchases. They'll be wanting to add a receiver, some batteries, spare props etc.