Straight to checkout with commerce
The way to programatically add a product to the cart and send a user to checkout would be to use the function:

function commerce_cart_product_add_by_id**($product_id, 
                        $quantity = 1, $combine = TRUE, $uid = NULL)


and 

function drupal_goto**($path = '', array $options = array(),
       $http_response_code = 302) 


to redirect the user to checkout.

Here is information on the functions:


commerce_cart_product_add_by_id
drupal_goto