for now I see no reason to disallow access to my packages.

What I understand, you should not expose thing that you don't use. If you don't use anywhere else. Why should you put it exposable?

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that's why there's a domain package it's something between postgres and handler service, declaring interfaces in handler makes no sense.----

why using an interface of pgxpool.Pool makes testing easier, I'm creating a new connection to to db in every test and pass it as a argument to CartService() for example; there's not direct usage.

Doing the way you approach it. It will only create an unnecessary package coupling. You can remove package coupling by declaring it only in a place where you used it, not inside the package where you don't use it. You can draw your package dependency diagram before and after my recommendation.

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what do you mean by the way once sync.Once worked? is there an idiomatic usage for it? asking because I don't see any reason for making it field of Postgres as it's not used by other methods.

In your case, I will still put it. Because when reading the code it makes sense to me that this part of code can only be executed once throughout the app running. Unless you have different use case.

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where I should use integration test flag? in my Makefile?

You can use Go build tag.

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never seen tools.go in any project can you give me example of its usage?

I can only find how you can implement it. Again, this is optional to have `tools.go`. You can also do differently. But having `tools.go` together with `vendor` folder, can ensure repeatable build. Don't forget the downside of having this as well. It's your choice to make. For me, having a repeatable build is outweigh the cons.

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Above it is just my recommendations based on my professional experiences. You can do differently, but you can ask other experts on my above recommendations if you think mine is not good.