To get more than one item to add to the cart in a single interface is much more advanced than adding a single item and may require a bit of custom coding. Here is the general idea, you can have the checkboxes be used to filter a recordset where items details are selected. Once you have this recordset in place you can use the add to cart from recordset server behavior to add these values to the cart including the id description, and price. You would then need to modify this add to cart to use the posted steps 2 and 3.
If you have DataAssist you can add in a DataAssist Search server behavior, this will build up the recordset based on the checkbox values for you. Once that is in place you will add in the add to cart from recordset server behavior and finally update the add to cart to use the posted values from steps 2 and 3.
There are other ways this can be done but they are all more custom and would require more hand coding.