Sometimes credit card companies will decline charges if the amount is more than you typically spend. If you give them a call at the number on the back of your card they should be able to pre-authorize the charge. I would recommend doing that before trying to complete the order form again.
If that doesn't work please send an email to firstname.lastname@example.org explaining this and include a screenshot of what you are seeing when you try to put your card in and our techs will be able to figure out what's going on. Thanks!