We process credit card payments through Stripe, which is also used by millions 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, it goes directly from your system to theirs without passing through Pleco's server.
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 sell it or give it to anybody else absent a court order, and we won't add you to our mailing list unless you separately go to this page to sign up for that. (if you'd like us to remove your email from our order retrieval database after you purchase, send us an email at support@pleco.com and we'll be happy to help)