You can use a credit/debit card or your PayPal account to subscribe to Noteflight Premium.
To switch payment methods select the profile icon in the top right corner of any page and choose "Account," or visit the link here https://www.noteflight.com/account#/details
From your Account page navigate to the Subscription section:
Under Billing select "Edit Billing":
From there you can select "PayPal" or "Edit" to switch your credit card:
Choosing PayPal will bring you to your account login to connect with Noteflight: