There is a known issue where under very specific circumstances customer trying to purchase are presented with an error when they input their Shipping Address Details. It looks like this:
 

The circumstances under which a customer would see this message are:

1. Based on a customer's IP address, the Topspin application thinks they are outside of the US and auto-fills the country it thinks they are in.
2. When the customer changes the country to United States, it throws the error.
3. The customer can then continue the purchase, though admittedly most would likely not do so.

The customer is able to continue with their purchase, but should take extra care in verifying their address on the following step of the purchase flow. They will be able to complete their purchase without further issue.