Hello Wojciech,
As someone who is relatively new to the world of Shopify apps, I am impressed with the quality of many of these apps, not to mention their simple accessibility.
One app that deserves special recognition is Dropifi. The website is available here, and this app provides a merchant's store with a simple contact widget in which customers can provide instant feedback to the website. As many comments have noted both here and elsewhere, one of the drawbacks of e-commerce is a lack of personal interaction between a merchant and a customer. Dropifi ameliorates this by helping bridge this gap; through the collection of intimate social feedback.
This tool helps a merchant analyze the emotions and demographs in messages, delivering feedback that is more individually catered and specific. This app is also compatible with different social media platforms, such as Twitter and Facebook (makes talking on the toilet THAT much easier!)
Again, I still consider myself a Shopify 'newbie', but I'd highly recommend this app to anyone who values great and personalized customer service. Did I also mention it is simple to use?