Problems checking out usually are the result of either browser problems or payment issues.
The shopping cart uses "cookies" in your web browser to keep track of the products you've ordered (they are just little files that keep track of information). Sometimes a cookie is not properly cleared by the browser and will create these sorts of problems. Often the type of problem you're seeing can be corrected by deleting the cookies from your browser and also clearing the "cache" or history. If you do this, most likely the problem will go away and you'll be able to place the order.
If you are using a credit card, make sure your account is not over limit. Many banks will deny transactions if the account is over limit. Also, make sure the address entered in the cart matches the billing address of the card you're using.
If you're using PayPal, make sure you have sufficient funds in your PayPal account to complete the purchase.
If none of the above work, feel free to send us a help request using the form on the "Contact Us" page. Select "Problem Placing a New Order" from the "Regarding" drop-down field.