Zagomail charges only for active and unique subscribers, so you are not charged if you have duplicated subscribers in multiple lists.
To add your billing details (credit card) you need to go to account > billing or Click here
To add a new credit card, just click on the button "Add card". It will be your primary card by default. If you want to use multiple credit card then you can add new card and make the one you want as a "default".
If you want to upgrade/downgrade your plan, just click on upgrade/downgrade.
In case you want to cancel your subscription, you have to click on the icon delete next to your subscription. The same if you want to remove your card.