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I see you have already cancelled your subscription, please note that it will remain in Economy plan up until September 16, 2015. On September 17, your account will switch to "Free" plan and you will not be billed anymore further.
When you upgrade or subscribed to our paid plans, the credit card details you used are securely processed on a 3rd party payment processor, in this case its Stripe, we used API to integrate and processed your payments. When you cancelled your subscription and is permanently deactivated, the credit card details won't be used anymore by Stripe to charge you, unless you upgrade again to a paid plan.
But let me forward this to our developers to have more information on this, we will update you on this thread.
JotForm Support Manager
The credit card information is not retained in our servers, so the information travels directly to the payment processor via their API.
It seems you had a previous Premium subscription via FastSpring which was canceled once you upgraded to Economyvia Stripe. So there is nothing you should worry about.
If you have further questions, let us know.