Circleboom's Servers uses Cloudflare which is an immensely popular internet security services provider to protect it's visitors, users, and subscribers. All pages are SSL protect by Cloudflare.
Circleboom uses industry lead payment operator Stripe as it's payment processor. Stripe is one of the top credit card processors of the World. (https://www.wired.co.uk/article/stripe-payments-apple-amazon-facebook)
On the upgrade page we display Stripe's credit card subscription form, and they do the job. All credit card information is stored on Stripe's servers securely. We do not see credit card details during or after the payments.
You can see Stripe’s logo lower left corner at the upgrade page.