Your select list should pass the Item ID, you then create a recordset that is filtered on the ID passed by the select list.
use the add to cart server behavior. Go to the serer behaviors panel, click the plus button and select eCart -> Add to cart -> Add to Cart. set the trigger to occur if the recordset is not empty. On the bindings panel, click the lightning bolt next to value for each cart column and bind it to the corresponding recordset column.