I would've thought of it this way; Is the user bringing a cart (/user/<USER_ID>/cart/<CART_ID>)? Is he bringing his own cart (/user/<USER_ID>/cart)? Is the s

I would've thought of it this way; Is the user bringing a cart (/user/<USER_ID>/cart/<CART_ID>)? Is he bringing his own cart (/user/<USER_ID>/cart)? Is the store supplying a cart for the user (/cart)? Applying real world objects and thinking, makes more sense in my mind. The third option instinctively is the most suitable way to go (IMO).