Hi , I too experienced the same difficulty as detailed above
checking the error logs :
database error [13-Dec-2015 01:14:53 UTC] WordPress database error Column ‘pnp’ cannot be null for query INSERT INTO wp_wpsc_cart_contents
(prodid
, name
, purchaseid
, price
, pnp
, tax_charged
, gst
, quantity
, donation
, no_shipping
, custom_message
, files
, meta
) VALUES (130, ‘DVD 10’, 768, 19.950000, NULL, 0.000000, 0.000000, ‘1’, 0, 0, ”, ‘N;’, NULL) made by require(‘wp-blog-header.php’), require_once(‘wp-load.php’), require_once(‘wp-config.php’), require_once(‘wp-settings.php’), do_action(‘init’), call_user_func_array, wpsc_submit_checkout, wpsc_cart->save_to_db, wpsc_cart_item->save_to_db
the ‘pnp’ entry is null and causing th insert to fail.
I added a simple
if($shipping == null) $shipping =”;
to line 511 in the cart-item.class.php just before the function call .
I suspect there may be a more correct/elegant solution ?
Damian