If you are using an IOS device (iPhone, iPad or iMac/MacBook) you many need to allow “Cookies” and turn off “Prevent Cross-Site Tracking” on your device. This can be done by:

Settings > Safari > Privacy and Security > Block All Cookies (turn this off)
Settings > Safari > Privacy and Security > Prevent Cross-Site Tracking (turn this off)

Safari > Preferences > Privacy (uncheck “Block all cookies & Prevent cross-site tracking”)

You can enable/disable these features temporarily while you make your payment, then re-enable afterwards to safeguard the rest of your browsing experience.