As per your requirement if you just want to get the customers comment on particular product in order summary then you don't need to use any app, you can do this by using custom fields also.
Using https://ui-elements-generator.myshopify.com/ , you can create an custom field on the product page where customer can add there comments.
If you want to hide this on cart page then can also hide it there.
You will receive this comment in order summary in backend once the prder is placed.
Nick here from Shopify. Great question!
There are a number of good message apps you can look into which can be in the corner of your store allowing customers to message you about everything and anything, not just necessarily the product on the page. What you're looking for specifically might be quite customized but there are a few different live chat message apps from the Shopify App store you can look into. Let's take a look at some of them below:
A lot of these apps have free plans or are free also.
Hopefully this helps!
All the best, Nick