Thanks for your reply! Fortunately, I solved my problem like 5 minutes ago. I set up a weight for a shirt, like 1kg (which is not of course ahah) and 1 shirt is 10$ int'l shipping. And 2.1 to up 3kg is 11$, which is then for 2 shirts. 3 shirts will be 3.1 to 4kg and so on.
I uploaded the New Standard theme so the shipping rates show up before filling up all the payment info. And it seems to works fine when I test it! :)
I saw a thread from 2006 asking for this additional item option, and a Shopify employee replied they will work on this in a near future. It is still missing and I guess lots of customers would be grateful for that option in your shipping settings! ;)
Happy holidays to you!