When we receive a gift, we feel a need to reciprocate.
Give things away for free. A free account, a free report, a free workshop, a gift card etc. Preferably something that people won't expect to get for free. People will feel the need to do something in return, whether it's buying something at your shop directly or referring friends (which you can make easy with social media integration). Note: if your competitors are also giving things away for free then people might also expect this from you and it's no longer a real gift.
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.