We process credit card payments through Stripe, which is also used by thousands of other companies (many of them a lot bigger than Pleco) and offers state-of-the-art security; even we have no idea what your credit card number is.
Email: we use your email address to send you a confirmation email with your purchase details. We also keep it in a database so that you can log on to retrieve your order information in the future if you lose it. These are the only things we use your email address for; we won't add you to our mailing list unless you separately go to this page to sign up for that.