[jmyeet]

It says "experiments show" but I'd need to know the methodology.

There is a real risk of selection bias here in that you only send handwritten thank you notes to your biggest, most loyal or most important clients. There is a line in this about it's only effective with clients who have some brand loyalty. So did the experiment account for this?

I'm reminded of of the "we value your opinion" or (worse) "you've been selected" for a survey. Surveys are zero cost of the Internet so I haven't been carefully chosen. That survey has simply been sent to everyone. It feels inauthentic.

If I buy a lime juicer and get a hand written thank you note it would similarly feel inauthentic.

High value clients tend to be high touch. It's why big ad spenders have account managers. High touch can take many forms. I'd be wary of simply trusting handwritten thank you notes in a vaccum.