Thinkspace is typically billed on a monthly basis through your credit or debit card. We also support ACH or eCheck. You can submit this card information to us directly right in your website's admin. If you've been notified about a declined transaction or you have a new card you'd like you use, head over to the admin and in the top right corner click Payments
If you're a new customer or don't have access to your website, you can also go to https://think.formstack.com/forms/cc
For another other billing questions you can contact us by submitting a support ticket or emailing us at billing@thinkspacehq.com