We are likely to value third-party consultation as objective, confirming, and without motive.
When we read reviews of 3rd party experts/ consultants, we value that information as being more objective and trustworthy. So besides testmonials of your customers (see Social Proof), can you get some external experts to review or talk about your services on conferences or their blogs? Even when you pay them to write a review on their blog and tell the audience it'll still work (example: sponsored review on Yoast.com). Of course it helps a lot if the review is positive ;).
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.