You're not texting credit card data; in fact, they're not handling it at all. From the FAQ:
Q: How do you guarantee that my credit card number is safe and secure?
We will send you a 128-bit encrypted HTTPS link via SMS that you can click to enter your credit card number. We do not store your credit card number. All payment processing is handled by Stripe.
Q: How do you guarantee that my credit card number is safe and secure?
We will send you a 128-bit encrypted HTTPS link via SMS that you can click to enter your credit card number. We do not store your credit card number. All payment processing is handled by Stripe.