Credit card purchases made online go through a bunch of authentication processes between the service you're purchasing and your bank/credit card company. The parameters they check are constantly in flux as a way to prevent credit card fraud. (This is also why a card that worked presently at a webstore might get denied later on and vice versa.) If any one of the parameters the bank is looking for in authentication doesn't meet their expectations, they decline the charge as a safety precaution. This isn't a Roll20 exclusive problem, but an ecommerce problem. Here's Stripe's (Stripe is what we use for ecommerce) website about credit card declines - <a href="https://stripe.com/help/declines" rel="nofollow">https://stripe.com/help/declines</a> What you might end up having to do is to contact your bank to ask them to allow purchases made from Roll20 go through.
