We occasionally have a customer ask why they can use a credit card at a local brick-and-mortar store, or to order goods online, but have trouble using the same card in our system. In some cases, this is because our anti-fraud measures stop the transaction. We use a 3rd party provider to help us calculate risk and many measures to identify fraudulent orders, including default limits on order dollar amounts, restrictions on credit cards whose address does not match the GeoIP address of the customer, and restrictions on the number of transaction attempts allowed per day.
These extra precautions exist because fraud is more common online overall, we can't perform other checks that physical transactors can (we can't inspect the card, look at ID, or require a signature, for instance), and we aren't at all as well protected from chargebacks because we can't provide a tracking number showing that a product was shipped.
If our system blocks you in some way that might be due to an anti-fraud measure, please contact us, and we can discuss additional materials that you can provide to prove your identity so that we can remove these. Alternately, if you're using a credit card for your purchase, you might consider trying Paypal.