The feeling of giving something up is stronger than the feeling of acquiring it.
People like to gain. But people hate to loose. Some psychological studies even show that losses are twice as powerfull as gains. So instead of persuading people to buy your products in terms of what they have to gain, try to persuade them in terms of what they will loose when they don't. (Further reading: Loss Aversion, Wikipedia)
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.