Yes that is correct, is hard to determine what page or url each site will like to use. mysite.com/cart is optional if someone wants to add a link to their theme or menu but it behaves just like the gallery cart link.
The problem is that it does -not- behave the same.
If one clicks on the mysite.com/galleries/mygallery-123/shopping-cart/ one only sees the images added to the cart for -that- gallery… not the ‘global’ cart.
So a user could check out only seeing the images from the one gallery.