what it does:
normally, the cart gets emptied when an order has been successfully executed.
setting this constant to “true” will stop this from happening.

might be useful when developing and checking orders from end to end to not have to re-add items to cart only to see how emails would look for example. Should probably not be used for production sites.

affects:
frontend only

usable:
anytime, can also be deleted again at any time.

example:

define('WPPIZZA_DISABLE_CLEAR_CART',true);/*bool*/

where to add/define:

in your wp-config.php before:
/* That's all, stop editing! Happy blogging. */

Disclaimer:
As with all filters, actions and constants:
Although these should be safe to use as indicated, there are no guarantees. Use at your own risk. Make backups before usage.