When using Paypal as a payment provider, the checkout process is unable to complete the transaction.
Paypal is no longer accepting transactions from sites that use SSLv3 due to security concerns. For more information, see https://www.paypal-community.com/t5/PayPal-Forward/New-Information-as-of-December-2-2014-POODLE-SSL-v3-Technical/ba-p/902239 .
The workaround is to use the ServicePointManager.SecurityProtocol Property on your checkout page to force the use of TLS instead of using SSL. An example would be:
Namespace: System.Net System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;
This forces TLS instead of SSL V3.