If you run a Cayan charge card transaction through the system, it will remember the card information so that you need not enter it again.* If you notice that this is not happening for some or all of your charges, there is a likely explanation.
Edit the patient information for that patient (display the patient in the top half of the screen, then click "Edit Info"). Scroll down and look for an item about halfway down that says "OK to keep charge card info in secure file?" That question must be answered "Yes" in order for the card information to be stored.
*Technically, it doesn't store the card information itself, but a "token": a representation of the card that is useless if stolen. This keeps the system "PCI compliant."