We look for ways to exert influence or control over a situation.
Our level of stress declines and feeling of certainty increases when we perceive greater autonomy. How larger the group, how larger this urge to make our own choices. You can introduce - even trivial - choices by introducing customization options. This can be applied to the product you sell (color, form, print, gift wrap) or your shop itself (different fontsize, another stylesheet). PS: don't offer too much choice to prevent Choice Paralysis
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.