I'm also having this issue. I love squarespace as a platform to use, but to sell an 'ecommerce' platform into the EU without this functionality is incredibly disingenuous. Especially for a €30+ cost per month. At the very least squarespace need to make this information very clearly visible before hours of development time have been wasted.
If you go on squarespaces website the products they use in example websites would fall under different tax rates in Germany, so if you used the website in the way they are advertising you would be breaking the law...
Everything on these forums points to this having been an issue for a long time, it leaves a bit of a bitter taste.