If you do not want to purchase products on Boom Learning via Paypal, you can also enter your credit card information directly. Your payment information is stored on using external security service called Stripe. Stripe's security is audited and it certified to PCI Service Provider Level 1, the most stringent level of certification available in the payment industry. Click this link to learn more about Stripe on their website.
To add a card, go to the Menu in the top right, then click My Settings. Then click the Membership tab to see your payment information. The screen you are looking for look something like this:
Simply click Add Card and enter your information to store it with Stripe, allowing you to use it to make purchases on Boom Learning.
To remove your card, simply click the red button that appears once you have added a card.