The problem could be because of your browser cache, DNS cache, or Internet Provider cache.
Please test the following procedures to see if any of these would solve the problem.
1) Please try using a different browser (Chrome, Safari, Firefox, Internet Explorer, etc) than you normally use.
This will eliminate problems with browser cache.
2) If you have a mobile phone that allows you to tether, please try connecting to the Internet via mobile rather than home Wifi.
This will eliminate problems with DNS and IP cache.
3) Please log out from your account and try placing the order again. You may log in again at checkout.
This will eliminate your account related problems.