High Traffic

Seems like a lot of purchases within a pretty short amount of time.

You would probably want to make sure that you clear out your options table so it does not fill up with sessions and transients records.

http://www.stumiller.me/does-wordpress-delete-expired-transients-from-the-database/
https://github.com/Seebz/Snippets/blob/master/Wordpress/plugins/purge-transients/purge-transients.php