Checkout and empty carts
Session issues
If your cart is emptying between the cart and the checkout, it may be another plugin or even the theme that is causing an issue.
Try deactivating all other plugins and switching to the WordPress default theme. If this works, re-enable the plugins one by one until you find the culprit and/or switch back to your old theme to see if this re-creates the problem, again.
If it doesn’t work, it could be a host issue. Possibly the session.save_path was not set up correctly in the php.ini. Try the following to see if it helps:
# memory_limit = 64M
# [uncomment - delete ";" - and set session.save_path to something like]:
# session.save_path = "/home/users/web/.../USERNAME/tmp"
(change path to suit your hosting)
It’s also been reported that turning off session.auto_start may also help.

FlipFlop 3.0