Thank you for your answer.
I understand that possibility provided by Squarespace for the USA customers,
But in their help about API I found this( please see image attached).
Based on that information, I was believed it's possible to integrate third party shipping software to calculate shipping at real time.
If that's not a case - I will be leaving Squarespace soon. As I don't see my self calculating manually shipping for the customers every time.
Off core, I would be able to use a flat rate, but that have two problems because ship my products are costly from £40 in the UK and £ 90 into the USA - can become very expensive if multiple products are Bought at once for customers, and that putting customers off from buying at all.
If a flat rate would be setted up for multiple products - it will be very costly for me.
Anyway, thank you for your answers.