Peter Keane wrote:
>
> 3. add a link to a cart resource pointing to the product
>
Do you mean like this?

GET /cart
"cart" : {... , "products": [], .... }

adding products:

PUT /cart
"cart": {..., "products": [ "/products/143", "/products/534" ], .... }

removing product 534:

PUT /cart
"cart": {..., "products": [ "/products/143" ], .... }